
Class ^J^'fe^ 

GopigteE" 



COPYRIGHT DEPOSm 



The D. Van Nostrand Company 

intend this book to be sold to the 
Public at the advertised price, and 
supply it to the Trade on terms which 
will not allow of reduction. 



ELECTRICITY 

EXPERIMENTALLY AND PRACTICALLY 
APPLIED 



A BOOK FOR THE BEGINNER AND FOR 
THE PRACTICAL MAN 



PRINCIPLES, EXPERIMENTS, PRACTICAL 
APPLICATIONS AND PROBLEMS 



BY 

SYDNEY WHITMORE ASHE, B.S., E E. 

Author of* Electric Railways"; Formerly Instructor in Physics 
and Electrical Engineering, Polytechnic Institute of Brooklyn 




NEW YORK 
D. VAN NOSTRAND COMPANY 

23 Murray and 27 Warren Streets 
1910 



,\p 



A<y, 



% 



Copyright, 1910, by 
D. VAN NOSTRAND COMPANY. 



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^C^ 



^ 



Cl.A•-^7l^■>«<'' 



DeMcateD 

TO 
MY WIFE 

HATTIE BELL LIPPENCOTT ASHE 

IN APPRECIATION OF HER SYMPATHY WHICH MADE THE 

PREPARATION OF THIS BOOK A PLEASURE 

AND NOT A TASK 



PREFACE 

Although a number of excellent works on practical 
electrical engineering are already in use, the author believes 
there is a place for a volume that will present the subject 
from an experimental standpoint. For many years the 
author has taught the subject of electrical engineering 
by means of experimental lectures and laboratory work to 
large numbers of practical men. Since these students 
have often lacked mathematical training, it has been nec- 
essary to explain the subject-matter in a very simple way, 
showing, wherever possible, its practical features. The 
same method of presentation has been used in this text, 
and it is hoped that a much larger audience may thus 
be reached and benefited. This volume is particularly 
adapted to courses of instruction given to those engaged in 
actual electrical work ; it should likewise prove useful as 
a text for high schools and for college laboratory courses. 

Owing to the manner in which the experiments are ar- 
ranged in the text and the detailed descriptions given this 
volume should prove helpful to the self-taught individual. 

The author has assumed at the outset, as in his lec- 
tures, that the reader is familiar with certain practical 
terms. This assumption is justifiable, since most beginners, 
if at all interested in their subject, are likely to have 
acquired a knowledge of general facts before they have 
begun the systematic study of the subject. Practical men 
in particular have a much greater knowledge of funda- 



VI PREFACE 

mental principles than they are ordinarily given credit for. 
Later in the text such terms are defined in detail. The 
explanations of experiments usually give the details for 
their performance before an audience, as experiments thus 
performed are much more difficult than those made in a 
laboratory. If any experiment is to be performed by a 
single individual, however, as in a laboratory, the author 
has trusted to the reader's originality to omit superfluous 
apparatus. 

In looking over the experiments it may be noted that 
a number of them are modifications of standard experi- 
ments such as those on induction developed by Faraday. 
There is, however, a large number of new experiments 
given. These experiments were developed in connection 
with educational courses given to the employees of the 
Brooklyn Rapid Transit Company, the Edison Illuminat- 
ing Company of Brooklyn, the New York Edison Com- 
pany, the Boston Edison Company, and the Consolidated 
Gas Electric Light and Power Company of Baltimore. 

The author's thanks are due to Mr. S. R. Keyes, of the 
Boston Edison Company, and to Dr. Frank W. Chandler 
at the Polytechnic Institute of Brooklyn for many valuable 
suggestions. To Professor Robert Spice and to the late 
Professor William A. Anthony the author is especially in- 
debted for assistance and inspiration afforded to him in 
his student days by their experimental lectures. It is to 
them that he owes his realization of the importance of 
such lectures in practical education. 

Sydney Whitmore Ashe. 

Brooklyn, N.Y., 
May, 1 910. 



TABLE OF CONTENTS 

CHAPTER I 
Magnetism 

PAGES 

Lodestone. Magnet. Earth as a Magnet. Horseshoe Magnet. Molec- 
ular Theory of Magnetism. Magnetic Induction. Effect of Tem- 
perature upon Magnetizability. The Effect of Vibration upon 
Magnetizability. The Obtaining of a Hysteresis Cycle. How to 
plot the Hysteresis Curve. Magnetic Field. A Magnet Pole travels 
along Lines of Force. Mayer's Needles. Practical Applications 
of Permanent Magnets. Ammeters, Voltmeters, Relays, Thomson 
Recording Wattmeters, Telephone Receivers. Questions . . 1-24 

CHAPTER II 
Electro-magnetism 

The Magnetic Field around a Straight Wire carrying a Current. The 
Compass Needle always tends to set itself Parallel to the Lines of 
Force. Effect of Current in a Coiled Wire. A Large Coil for 
Experimental Purposes. Limit Switch, Overload Relays. The 
Electro-dynamometer. Permeability, Saturation of Iron. The 
Electro-magnet. The Blow-out Magnet. Applications of Electro- 
magnetism. Electric Bells, Buzzers, Relays, Sounders. The Elec- 
tric Motor. Thomson Inclined Coil Ammeters and Voltmeters. 
Thomson Inclined Coil Wattmeter. Weston Indicating Wattmeter. 
Traction Electro-magnets. Questions 25-45 

CHAPTER III 
Electro-magnetic Induction — Theory of the Dynamo 

Faraday's Discovery. Relation of Turns, Flux, Speed of Coil to E. M. F. 
Generated. Generators. Motor Generators, Double Current Gen- 
erators, Rotary Converters, Balances. Variation of E. M. F. of 



viii TABLE OF CONTENTS 

PAGES 

Generator. Magnetization Curve of a Shunt Dynamo. Residual 
Magnetism. Mutual Induction. Foucault or Eddy Currents. Ap- 
plications of Foucault Currents. Practical Applications of In- 
duction. Rumkorff Coil, Telephone Circuit, Wireless Circuit. 
Wireless Sending and Receiving Apparatus. Questions . . 46-58 

CHAPTER IV 

Ohm's Law 

Electro-motive force, Current, Resistance. Resistance Defined. Ohmic- 
values of Resistance. Circular Mil. Resistance of Copper Con- 
ductors. Temperature Coefficient. Wire Table. Electro-motive 
Force Defined. Standard Cells. Carhart-Clark Cell. Weston 
Cell. Current Defined. Values of Current. Ohm's Law. Appli- 
cations of Ohm's Law. Example. Questions. . . . 59-76 

CHAPTER V 
Primary and Storage Batteries 

The Simple Cell. Galvani's Experiment. Chemical Action of Cell. 
E. M. F.'s of Cell. Effect of Changing Electrolytes. Polarization. 
Effect of Changing Electrodes. Depolarizers. Electrolytic De- 
polarizers. Electrolytic Condensers. Closed Circuit Cells. Daniell 
Cell. Local Action. Gravity Cell. Plating of Copper. Bunsen 
and Grove Cells. Grenet Cell. Open Circuit Cells. Leclanche 
Cell. Dry Cells. The Storage Battery. Theory of the Storage 
Battery. Operation of a Storage Battery. Rating of Cells. How 
to charge a Storage Battery. Edison Cell. Types of Commercial 
Cells. Questions 77-100 

CHAPTER VI 
Electrolysis 

Electrolytic Corrosion. Definition of an Electrolyte. Positive and 
Negative Temperature Coefficients. Decomposition of Acid Solu- 
tions. Electro-chemical Equivalents. Hoffman Voltameter. 
Electro-chemical Equivalent of Hydrogen. Polarity Indicator. 
Metallic Salt Solution. Electroplating. Copper Plating. Gold 
Plating. Silver Plating. Nickel Plating. Brass-plating Solution. 



TABLE OF CONTENTS ix 

PAGES 

Plating E. M. F.'s. Critical Current Density. Electrolytic Products. 
Alkalies and Bleach. Sodium. Aluminium. Potassium Chlorate. 
Sponge Lead. The Electric Furnace and its Products. Graphite. 
Calcium Carbide. Carborundum. Barium Hydrate. Miscellane- 
ous Substances. The Electrolytic Rectifier. Electrolytic Inter- 
rupter. The Pail Forge. Questions ..... 101-118 

CHAPTER VII 

The Three-wire System 

Two-wire System. Three-wire Edison System. Three-wire System with 
Generator. Three-wire System Converter. Theory of the Three- 
wire System. Experiment on Three-wire System. Old and New 
Style Branch Circuits. Questions ...... 1 19-124 

CHAPTER VIII 
Electrical Measurements 

Ammeter and Voltmeter Method of Measuring Resistance. Measuring 
Resistance of Voltmeter. Voltmeter Method of Measuring Resist- 
ances. Comparison of Resistances with Voltmeter. Calibration of 
an Ammeter. Lamp Boards. Calibration of Ammeters. Series 
Method. Calibrating Rotary Ammeter. Calibrating Feeder Am- 
meter. Calibration of a Voltmeter. Potentiometer Method of 
Calibrating a Voltmeter. Calibration of an Indicating Wattmeter. 
Wheatstone Bridge Method of Measuring Resistance. Post-office 
Box. Slide Wire Bridge. Wire Roller Bridge. Resistance of an 
Electrolyte. Carey-Foster Bridge. Thomson Double Bridge. In- 
sulation Test. The Galvanometer. How to set up a Galvanometer. 
Determination of Resistance of Galvanometer. Determination of 
the Constant of a Galvanometer. Measurement of Capacity. Com 
mercial Testing Sets. Questions 1 25-1 51 

CHAPTER IX 
The Shunt Motor 

The Shunt Motor. Testing out Circuits. Measuring Armature and 
Field Resistance. Magnetic Circuit of Field Coils. Tests for Im- 
proper Connection of Field Coils. Magnetic Circuits of Armature. 



TABLE OF CONTENTS 



Magnetic Circuits of Armature and Field. Neutral Plane. Chang- 
ing Direction of Rotation. Operating Connections. Armature 
Circuit. Counter Electro-motive Force. Starting Boxes. Direc- 
tions for Connecting up a Shunt Motor. Magnet Arm on Starting 
Box. Overload Release. Changing Direction of Rotation. Speed 
Variation. Rheostats. How to tell v^hether Field Resistance is 
All In or All Out. Theory of Speed Variation. Location of Trouble. 
Tests for Grounds. Tests for Short Circuits. Resistance of Ground. 
Interpole Motors. Questions 152-173 

CHAPTER X 

The Series Motor 

Characteristics of the Series Motor. Magnetic Circuits of the Series 
Motor. Resistance of Armature and Field Circuits of the Series 
Motor. Starting of a Series Motor. Speed and Tractive Effort 
Curves. Relation of Torque to Tractive Effort of Series Motor. 
Relation of Current Input, Speed, Torque, and Voltage of a Series 
Motor. Changing Direction of Rotation. Railway Controllers. 
Tw^o and Four Motor Equipments. Bridge Control. Multiple 
Unit Control. Emergency Brake of the Series Motor. Testing out 
Controller. Structural Features of Motor. Questions. . 174-185 

CHAPTER XI 

The Arc Light 

The Carbon Arc. Spectrum of Arc. Physics of Carbon Arc. Intrinsic 
Brightness of lUuminants. Efficiency of Light-giving Bodies. The 
Ultra Rays. Arc Lamp Circuits. Alternating Arc Circuit. In- 
closed Arcs. Recent Developments in Arc Lamps. Factors to be 
considered in Selecting an Illuminant. Holophane Reflectors. 
The Flaming Arc. The Magnetic Arc. Questions . . 186-201 

CHAPTER XII 

Incandescent lUuminants 

The Carbon Incandescent Lamp. Early Discoverers. Edison's First 
Lamp. Making the Carbon Filament. Treating the Filament. 
Mounting the Filament. Characteristics of Filaments. The Tan- 



TABLE OF CONTENTS xi 

PAGES 

talum Lamp. The Tungsten Lamp. The Tungsten Filament. 
The Moore Vacuum Tube. Feeder Valve for Moore Tube. The 
Cooper-Hewitt Mercury Vapor Lamp. The Nernst Lamp. Ques- 
tions 202-220 



CHAPTER Xni 

Recording Wattmeters and their Use 

Old Edison Bottle Meter. Thomson Recording Wattmeter. Armature, 
Armature Resistance, Compensating Coil, Field Coils, Magnets, 
and Gears of Thomson Recording Wattmeter. Type C Thomson 
Recording Wattmeter. Friction of Recording Wattmeter. Testing 
Meters. The Direct Method of Testing. The Rotating Standard 
Method. The Standardized Resistance Method. Load Box of 
Boston Edison Company. Installation Tests. Inspection Tests. 
How to make Tests. Method of Determining Normal Load. Mod- 
ern Meter Installations. Meter Wiring. Questions . . 221-241 

CHAPTER XIV 

Elementary Principles of Alternating Currents 

Definition of an Alternating Current. Comparison with Direct Cur- 
rent. Instantaneous Current Curve. Sine Curve. Generation of 
E. M. F.'s. The Contact Maker. Balance Set to obtain E. M. F.'s. 
Method of Balancing E. M. F.'s. Oscillograph. Effective Current 
Values. Form Factor. Alternating Current Generators, Various 
Types. Capacity. Effect of Capacity in an Alternating Current 
Circuit. Capacity and Current Relation. .Capacity Reactance. 
Series Parallel Combination of Condensers. Self-induction. Deri- 
vation of Capacity and Self-induction Formulae. Relations of Re- 
sistance and Inductive E. M. F.'s. Relation by Means of Vectors 
of Resistance and Inductive E. M. F.'s. Vectors. Vector Relations 
in Single and Three-phase Circuits. Trigonometric Expressions. 
Meaning of Sines, Cosines, Tangents. Values of Sines, Cosines, 
Tangents. Circuits containing Resistance, Inductance, Capacity. 
Mathematical Relation of Inductance, Resistance, and Capacity. 
Series and Multiple Circuits. Reactance and Admittance Defined. 
Power in a Single-phase Circuit. Power Factor. Determination 
of Power Factor. Power Measurements in a Three-phase Circuit. 



xii TABLE OF CONTENTS 

PAGES 

Tangent Formulae. Single-phase Induction Wattmeters. Poly- 
phase Integrating Wattmeter. Three-phase Circuit Current Lag- 
ging 30°. Questions . . . , 242-282 

CHAPTER XV 
The Alternating Current Transformer 

Theory of the Transformer. Relation of Transformer E. M. F.'s. Rela- 
tion of Instantaneous E. M. F.'s. Operation of Transformer. Effi- 
ciency and Losses of a 400-kw. and an 800-kw. Transformer. 
Construction of a Modern Transformer. Experiments on Trans- 
formers. Types of Transformers. Manhole Transformers. Station 
Transformers. Six-phase Connections. Ratio of Transformers. 
Questions 283-298 

CHAPTER XVI 
The Induction Motor 

Theory. Windings. Slip. Experiments showing Poles, Transformer 
Features, Rotating Field, etc. Operation of an Induction Motor. 
Starting. Changing Direction of Rotation. Care of Induction 
Motors. Questions 299-306 

CHAPTER XVII 

The Rotary Converter 

Theory of E. M. F. Relations, Methods of Starting. Starting from 
Direct Current Side. Starting by Means of Induction Motor. 
Starting from Alternating Current Side. The Hunting of a Rotary 
Converter. Synchronizing. Synchronizing with a Voltmeter. 
Synchronizing with a Frequency Changer. Crossed Phases. Mean- 
ing of Unity Power Factor. Converters operating in Parallel. Re- 
cent Developments in Converters. The Booster Converter. The 
Interpole Converter, Rotary Converters versus Motor Generators, 
Questions 307-327 

APPENDIX 

Experimental Projecting Apparatus 329-341 

Formulae . 342 



LIST OF ILLUSTRATIONS 

FIG. PAGE 

I, 2, 3. Magnetic Spectrum. Compass. Repulsion of Magnets . . 2 

4, 5. Attraction of Magnets. North Magnetic Pole .... 3 

6. Magnetic Pole . . . . 4 

7. Horseshoe Magnet from Weston Voltmeter . . . . . 5 

8. Molecular Magnets 6 

9. 10. Magnets made by breaking Magnetized Needle. Magnetic In- 

duction ........... 7 

11. Magnetization Loss with Heat 8 

12. Keeper and Magnet . 9 

13. Set-up for obtaining Hysteresis Curve II 

14. Hysteresis Curve 12 

15. Partial Hysteresis Curve 13 

16. 17, 18, 19. Magnetic Spectrum. Shaker for Filings. Path of Lines 

of Force. Cross Magnetization 14 

20 a. Set-up show^ing that Magnet Pole travels along Lines of Force . 15 
20b, 21. Small Electro-magnet. Molecular Magnets . . . .16 
22, 23, 24. Movement of Weston Standard Voltmeter. Weston Stand- 
ard Voltmeter Complete. Station Type of Weston Voltmeter . 17 
25, 26, 27. Laboratory Standard Weston Voltmeter. Weston Station 

Type of Ammeter. Magnets of Thomson Recording Wattmeter . 18 

28. Thomson Recording Wattmeter 19 

29, 30. Telephone Receiver (M. E. S. Co.). Weston Speed Tacho- 

meter 20 

31, 32. Weston Relay. Wattmeter Disc .21 

33. Weston Ammeter, Central Zero, with Shunt 22 

34, 35, 36. Magnetic Field around Wire. Presence of Field shown with 

Filings. Field shown with Compass ...... 25 

37, 38. Support for Compass for Projection. Neutralization of Magnetic 

Field 26 

39, 40. Wire carrying Current tends to encircle Magnet. Helix . . 27 

41, 42. Solenoid (termed Long Tom). Sucking Coil .... 28 

43. Limit Switch (Westinghouse) . . ... . . 29 

44. Electro-dynamometer 30 

xiii 



xiv LIST OF ILLUSTRATIONS 

FIG. PAGE 

45, 46. Magnetization Curve. Filings showing Strength of Electro- 
magnet 32 

47, 48, 49, 50. Nails showing Distribution of Flux of Magnet. Dia- 
phragm and Magnet. Tracing the Field. Principle of Edison 

Ore Separator • • ZZ 

51, 52, 53. Arc extinguished with Magnet. Arc extinguished with 

Electro-magnet. Electric Bell (M. E. S. Co.) .... 34 

54, 55, 56, 57. House Bell Circuit (M. E. S. Co.). Circuit of Electric 

Bell. Telegraph Line. Transmission Key (M. E. S. Co.) . . 35 

58,59,60. Relays (M. E. S. Co.). Sounder. Eectric Motor . . 36 

61, 62. Armature of Motor (Westinghouse). Field Coils of Motor 

(Westinghouse) 37 

63. Induction Motor (G. E. Co.) 38 

64, 65, 66. Commutator (Westinghouse). Series Winding. Rotation 

of Armature 39 

67, 68, 69, 70. Shunt Connection. Series Connection. Armature and 

Field Symbols. Cross-section of Armature (Westinghouse) . 40 
71, 72. Thomson Inclined Coil Ammeters (Section). Thomson In- 
clined Coil Ammeter 41 

73. Weston Wattmeter Movement ....... 42 

74. Tractive Magnet built by Cutler Hammer Clutch Co. ... 43 

75. 76, 77. Method of Generating Zero E. M. F. Lines of Force not 

Interlinked. Generating an E. M. F. (Magnet and Coil) . , 46 

78. Generating an E. M. F. (Coil and Coil) 47 

79. Rotary Converter (G. E. Co.) 48 

80. Motor Generator (Westinghouse) . 49 

81. 82. Motor Generator (G. E. Co.). Circuit of Motor Generator . 50 
83,84. Circuit of Balances. Field Circuit of Generator . . -51 
85, 86. Magnetization Curve. Excitation of Generator ... 52 

87. Principle of Induction 53 

88. Wattmeter Disc with Magnets 54 

89. 90. Principle of Eddy Currents. Insulation of Eddy Currents . 55 
91, 92, 93, 94. Ruhmkorff Coil. Shocking Coil. Telephone Circuit. 

Wireless Circuit . . . 56 

95, 96. Wireless Sending Apparatus. Wireless Receiving Apparatus . 57 

97. Desk Telephone 5^ 

98. Car Resistance .60 

99. Circular Mil 61 

100,101. Principle of Distribution of Potential. Carhart-Clark Cell . 66 
\02a,\oib. Weston Cell. Weston Cell (Interior View) . . 67-68 

103. Ohm's Law Illustrated 72 



LIST OF ILLUSTRATIONS XV 

FIG. PAGE 

104. Method of obtaining Low Voltage 75 

105, 106. Galvani's Experiment. Voltaic Cell 77 

107. Projecting Tank 78 

108, 109. Support for Electrodes. Support in Position ... 79 

1 10. Polarization Tank 80 

111. Carbon Cell 81 

112,113. Electrolytic Condenser. Electrolytic Cells in Series . . 82 

114,115,116. Gravity Cell. Copper Electrode. Crowfoot Zinc . . 85 

117. Plating of Copper 86 

118,119. Leclanche Cell. Zinc for Leclanche Cell .... 87 

120,121. Mesco Dry Cell. Red Seal Dry Cell 88 

122. Variation of Resistance of H2SO4 90 

123, 124. High Resistance of Pure Water. Conductivity of Pb02 and 

PbS04 Shown 91 

125. Battery Pellet 92 

126, 127, 128. Battery Discharge Curve. Testing Sulphated Plate. 

End Cells 93 

129, 13c, 131. Battery Charging Circuit. Test Hydrometer. Charging 

Circuit ............ 94 

132. Charging Circuit 95 

133, 134. Tudor Plates. Laboratory Chloride Cell .... 96 
135, 136. Telephone Chloride Cell. Box Negative .... 97 

137, 138. Exide Battery. Manchester Positive 98 

139, 140. Rolled Negative. Shelf Negative 99 

141, 142, 143. Conductivity of Sodium Acetate with Heat. Positive 

Temperature Coefficient. Negative Temperature Coefficient . 102 
144, 145. Electrolysis Projecting Tank. Decomposition of Acid Solu- 
tions ............ 104 

146, 147. Hoffman Voltameter. Electro-chemical Equivalent of Hy- 
drogen 105 

148, 149. Polarity Indicator ......... 107 

150. Critical Current Density Apparatus iii 

151. Wehnelt Interrupter 117 

152. The Pail Forge ii8 

153. 154, 155. Two-wire System. Edison Three-wire System. Three- 

wire System (Generator) 119 

156, 157, 158, 159, 160. Three-wire System (Converter). Principle of 

Three-wire System . 1 20-12 1 

161. Experiment on Three-wire System 123 

162, 163. Old Style Branch Circuits. Approved Branch Circuits . .124 
164. Ammeter-voltmeter Method of Measuring Resistance . . .125 



xvi LIST OF ILLUSTRATIONS 

FIG. PAGE 

165,166,167. Measurement of Resistance of Voltmeter. Weston Volt- 
meter Circuit (Station Instrument). Weston Voltmeter Suspen- 
sion 127 

168. Measuring Resistance with Voltmeter 128 

169,170. Showing Distribution of Potential. Standard Resistance . 129 
171, 172, 173, 174. Method of Connecting Contacts. Potential Taps. 

Lamp Board . . 130 

175, 176. Calibrating Rotary Ammeter. Calibrating Feeder Ammeter . 131 
177, 178. Calibrating Ammeters in Laboratory. Calibrating a Volt- 
meter ... 132 

179, 180. Leeds and Northrup Company Potentiometer. Circuits of 

Leeds and Northrup Company Potentiometer . . . -133 

181,182. Diagram of Potentiometer Circuit. Standard Cell Circuit . 134 

183. Indicating Wattmeter Circuit . . . . . . .136 

184, 185, 186, 187, 188, 189, 190. Calibration of Indicating Wattmeter. 

Principle of Wheatstone Bridge ^Zl''^!)^ 

191. Post-office Box 139 

192, 193. Galvanometer Shunt. Various Arrangements of Four-series 

Resistances . 140 

194. Slide Wire Bridge 141 

195 a, 195 b, 196. Wire Roller Bridge 142 

197, 198. Resistance of Electrolyte. Carey-Foster Bridge . . . 143 

199. Insulation Test . . 145 

200. Galvanometer 146 

201. Measuring Resistance of Galvanometer 147 

202. 203. Method of obtaining Low Potential. Low Potenial Obtained 148 
204, 205. Measurement of Capacity. Substituting Switch . . . 149 
206, 207, 208, 209, 210. Queen Slide Wire Bridge. Queen Acme Test- 
ing Set. Queen Decade Testing Set. Queen Laboratory Wheat- 
stone Bridge, Queen Wheatstone Bridge . . . . .150 

211. CQ. Motor installed on Ceihng . . . . : . .152 

212. Testing out Circuits of Motor •. 153 

213. Shunt Machine 154 

214. 215, 216. Magnetic Circuits of Motor. Testing out Magnetic Cir- 

cuits .... 155 

217. Magnetic Circuits of Armature 156 

218, 219. Changing Direction of Rotation. Measuring Field Current . 157 

220. To show Induction of Field Winding 158 

221. Starting of Shunt Motor 159 

222. Starting Box 160 

223. Starting Box Full On . . . 161 



LIST OF ILLUSTRATIONS xvii 

FIG. PAGE 

224. Starting Box (Westinghouse) . . ' 162 

225. Cutler Hammer Starting Box ....... 163 

226. Motor Circuits 164 

227. 228. Field Circuit. Remote Control Resistance .... 165 

229. Self-contained Rheostat (G. E. Co.) 166 

230,231,232,233,234. Dial of Rheostat. Circuits of Rheostat. Field 

Rheostat. Cross-section of Cutler Hammer Rheostat. Burnt out 

Rheostat 167 

235. Test for Grounded Armature . . , . . . .169 

236, 237, 238. Test for Grounded Field Coil. Test for Short-circuited 

Field Coils. Test for Short-circuited Armature Coils . . .170 

239. Test of Grounded Switchboard . . . . . . • 171 

240. Interpole Motor (G. E. Co.) 172 

241. Series Motor Circuit 174 

242. Measuring Resistances of Series Motor 175 

243. 244. Tractive Effort of Series Motor. Relation of Torque to Trac- 

tive Effort of Series Motor 177 

245. Characteristic Curves of Series Motor 178 

246, 247. Measuring Torque of Series Motor. Measuring Speed of 

Series Motor . . 179 

248, 249. Showing Relation of Speed to Current. Circuits for Changing 

Direction of Rotation of Series Motor 180 

250, 251, 252. Two Motors in Series. Two Motors in Parallel. Four 

Motors in Series .......... 181 

253, 254, 255. Four-motor Equipment. Four Motors in Parallel. Bridge 

Control 182 

256. Alternating Current Arc 186 

257. Method of obtaining Spectrum 187 

258. Temperature shown with Thermo Element 190 

259. 260. Iron Arc. Sign 192 

261, 262. Simple Arc Circuit. G. E. Arc Mechanism .... 193 

263. Both Carbons Down-feed 196 

264. Holophane Reflector . . 198 

265. Carbon Incandescent Lamp 202 

266. Squirting Filaments (Westinghouse Lamp Works) . . . 204 

267. Drying Filaments (Westinghouse Lamp Works) .... '205 

268. Bulb with Tip Added 207 

269. 270. Filament Mount. Bulb ready for Exhaust .... 208 

271. Tantalum Lamp 209 

272, 273. Tantalum Filament. Circuit illustrating Positive and Nega- 

tive Temperature Coefficient 210 



xviii LIST OF ILLUSTRATIONS 

FIG. PAGE 

274. Circuit to show Variation in Candle Power with Pressure . .211 

275. Tungsten Lamp .......... 213 

276. Moore Tube 214 

277. Feeder Valve for Moore Tube 215 

278. Cooper-Hewitt Tube 216 

279. Circuits of Cooper-Hewitt Tube 217 

280. Westinghouse Xernst Lamp 218 

281. 282. Glower. Circuits for Xernst Lamp . . . . .219 

283. Armature of T. R. W 221 

284, 285. Armature Resistance, T. R. W. Compensating Coil . . 222 
286, 287, 288, 289. Field Coil, T. R. W. Magnets, T)i)e C Meter. 

T. R. W. Gear 223 

290. Type C, T. R. W 225 

291. Parts of Type C, T. R. \V 224 

292. Spring Brushes, T. R. W 225 

293. Cover of Old Style T. R. W 226 

294. 295. Wattmeter Circuit . 227 

296. Friction Curves of Meters ........ 228 

297. Calibrating a Meter 229 

298. Standardized Resistance used by the Boston Edison Co. . . 231 

299. Load Box, Boston Edison Co. 232 

300. Service Test of Meter 233 

301. Field Coils of Meter improperly Connected 234 

302. Typical Meter Installation, Alternating Current Service . . . 236 

303. 304. Typical Service of Meters. Combination Direct Current and 

Alternating Current Service ....... 237 

305. T. R. W. and T\'pe C Meter Service 238 

306, 307. Typical Direct Current Service. Single A. C. Service . . 239 

308. Typical Meter Installation 240 

309. Instantaneous Current Curve . 243 

310,311,312. Sine Curve. Generation of E. M, F 244 

313, 314. Contact Maker 245 

315, 316. Using a Balance Set to obtain E. M. F. Curve. Method of 

Balancing E. M. F.'s 246 

317. Balance Apparatus 248 

318. Interior of Weston Alternating Current Voltmeter . . .251 

319. 320, 321, 322. Single-phase Generator. Three-phase E. M. F.'s. 

Three-phase Generator. Inductor Generator .... 253 
323, 324. Experiment showing Charging of Cables. Dialectric Polari- 
zation ....... 254 

325. Experiment showing Charging of Condensers with Direct Current . 255 



LIST OF ILLUSTRATIONS xix 



326. Capacity and Current Relations 256 

327, 328. Relation of Sides in Right Angle Triangle. Condensers in 

Series 258 

329. Condensers in Parallel 259 

330. Relation of Resistance and Inductive E. M. F.s . . . . 263 

331. 332, 333. Line E. M. F. and Counter E. M. F. Resistance and In- 

ductive E. M. F.'s. Alternating Current E. M. F. Curve . . 264 

334. Relation of Current and Inductive E. M. F.'s .... 265 

335, 336. Vector. Vector Relation of Inductance and Current . . 266 
337> ZZ^i 339j 340- E. M. F. and Current in Phase. Vector Relation of 

Current and Capacity Reactance. Y-connection for Three-phase 
Circuit. Relation of Transformer E. M. F.'s .... 267 
341, 342. Resistance alone in Circuit. Vector Relation of Resistance 

and Inductance 268 

343. Trigonometric Relations in a Right Triangle .... 269 

344, 345, 346, 347. Vector Relation of E. M. F.'s. Effect opposite to 

Fig. 345. Ohmic Relation of Resistance and Reactance . .271 
348, 349, 350. Parallel Arrangement of Reactances. Vector Relation 

Admittance. Power in a Single-phase Circuit .... 273 

. 274 



351. E. M. F. and Current in Phase .... 

352, 353. E. M. F. and Current out of Phase. Unity Power Factor . 275 

. 276 

• 277 

. . 278 



279 



354. Experimental Method of Measuring Power Factor 

355. Power Measurement in a Three-phase Circuit 

356. 357. G. E. Single-phase Induction Wattmeter 
358, 359, 360. Jewel Support of Wattmeter. Induction Wattmeter 

Vector Relation of Single-phase Meter E. M. F.'s 
361, 362. Vector Relation of Current and Potential Vectors in a Poly- 
phase Meter. Polyphase Meter Vectors, Current Lagging 30^ . 281 

363. Experimental Study of Transformer ...... 283 

364, 365. Simple Transformer. Relation of Transformer Vectors . . 284 
366, 367, 368. Relation of Instantaneous E. M. F.'s in a Transformer. 

Load connected to Transformer. Load connected to I : I Trans- 
former ............ 285 

369. Modern Method of Building Transformers . . . . .287 

370, 371, 372. Cross-section of Fig. 369. Queen & Co, Experimental 

Transformer. Showing Principle of Transformer . . . 288 
373» 374j 375j 376. Reluctance of Transformer Air Gap Shown. Study 
of Transformer. Varying Reluctance of Transformer. Measuring 

Copper Loss of Transformer 289 

377j 378* 379- Efficiency of Transformer. Manhole Transformer. Man- 
hole with Transformer Installed 290 



XX LIST OF ILLUSTRATIONS 

FIG. PAGE 

380. Waterproof Terminals for Subway Transformer . . . .291 

381, 382. Station Transformer Connections, Six-phase . . . 292-293 
383, 384. Type H Transformers i : 2 Ratio. Type H Transformer i : \ 

Ratio 296 

385, 386, 387. Ratios of Transformers. Neutralizing Self-induction of 

Transformer. Transformer Secondaries in Series . . . 297 

388. Transformer Secondaries connected in Opposition . . . 298 

389. Induction Motor 299 

390. Experimental Study of Induction Motor ..... 300 

391. 392, 393. Copper Sphere. Transformer Feature of Induction 

Motor Shown. Poles of Induction Motor Shown , . . 301 
394, 395. Progression of Poles of Induction Motor with Current. Two- 
phase Induction Motor 302 

396. Three-phase Induction Motor operated from Single-phase Circuit 303 

397. Induction Motor operating a Blower ...... 304 

398. Converter Armature 308 

399. Synchronizing with Lamps 315 

400. Synchronizing with Lamp connected to Transformers . . .316 

401. 402. Synchronizing Two Converters. E. M. F. Relations when 

Machines are out of Phase . . . . . . .318 

403, 404. Crossed Phases. Unity Power Factor of Converters . .319 

405. Experiment illustrating Unity Power Factor 320 

406, 407, 408. Booster Converter ....... 322-323 

409, 410, 41 1. Position of Zero Boost on Converter. Boost in a Positive 

Direction. Boost in a Negative Direction 324 

412, 413. Interpole Converter 325-326 

414. Experimental Lantern arranged for Horizontal Projection . . 330 

415, 416. Experimental Lantern arranged for Vertical Projection. Sin- 

gle Lantern 331 

417. Lantern Set-up 333 

418. Two Lamps in Series 335 

419. Automatic Arc Lamps . . . ZZl 

420. Combination Beseler Double Lantern and Moving Picture Ap- 

paratus 338 

421. 422. Weston Projecting Instruments. Combination Board used 

with Projecting Galvanometer 340 



EXPERIMENTAL ELECTRICITY 



CHAPTER I 
MAGNETISM 

Lodestone. — Lodestone is an oxide of iron, magnetitey 
FcgO^, which has the property of attracting small particles 
of iron and, when freely suspended, of assuming a position 
pointing north and south. It also has the characteristic 
of imparting its properties to a piece of steel that has been 
rubbed by it. It is quite heavy, of a black lead color, of 
considerable hardness, and is usually found in iron mines in 
various parts of the world. The name *' magnet" as applied 
to the lodestone, leading stone, not load stone, is derived 
through the Latin magnes or the Greek /j,ciyv7]^, meaning 
magnet. It is supposed that the Chinese used small 
pieces of this stone suspended as compasses two thousand 
years before the Christian era. A description of a primi- 
tive form of compass in use on the Syrian coast was given 
by Kibdjaki in 1242 a.d. In 1260, on the return of Marco 
Polo from Cathay, he brought a knowledge of the compass 
as used by the Chinese. The Italians give to Flavio 
Gioja the credit of inventing the compass in 1 300-1 320, 
but it is probable that he simply improved upon the old 
form of the instrument. 

Magnet. — If a piece of steel is placed in a coil of wire 
carrying an electric current, it becomes magnetized. This 
magnetic condition of the steel may be observed by 



EXPERIMENTAL ELECTRICITY 




Fig. I. — Magnetic Spectrum. 



placing over the steel a piece of paper and sprinkling it 
with iron filings, thus obtaining a 7nagiietic spectrum. 

In observing this spectrum it will 
be noted that the attraction of the 
magnet for the filings is greatest 
at both extremities of the magnet, 
the poles, and that it is practically 
zero at the middle of the magnet. 
Now, if we substitute for the 
paper a piece of thin glass. Fig. i, and repeat the experi- 
ment, we shall note that the power of the magnet to attract 
the filings still exists. This fact leads to the conclusion 
that — 

Magnetism cannot be Insjilated. 

Experiment i . Place a piece of paper over a magnet and obtain 
a spectrum. Note that the lines of the spectrum seem to pass out from 
one pole and enter the other pole as though this imaginary force 
traveled in curves. 

If a small magnet be pivoted, Fig. 2, so that it is free 

to move, it will assume a definite direction 

pointing north and south. When a magnet 

is thus mounted, it is termed a compass. 

Bringing another magnet near a compass 

needle so that the north pole of the mag- 
net is near the north pole of the compass, 

Fig. 3, a repulsion of the end of the compass 

needle occurs. The same effect will 
be noted if two south poles are 
brought near each other. If, how- 
ever, a north pole and a soutJi pole 
be brought near each other. Fig. 4, 
attraction of the compass needle 




Fig. 2. — Compass. 




Fig. 3. — Repulsion 
Magnets. 



occurs. 



MAGNETISM 3 

Like poles repel each other, and unlike poles attract each 
other. 

Experiment 2. Bring near a compass needle first the north pole 
of a magnet and then the south pole, noting 
the result. Bring one end of a non-magnetized n!^^^'^,^ 

piece of iron near first the north and then the _JX,^^V-^ 

south pole of a compass needle, and note that ^^^v/\^ 

the piece of iron attracts both ends. Also ^^fc^ 

note that either end of the iron rod has the ^ 

same effect on the magnet. Why? 

Earth as a Magnet. — A compass ^^ 

needle assumes the position pointing fig. 4. — Attraction of 
north and south because the earth Magnets, 

possesses the property of a magnet, having a magnetic 
north and a magnetic south pole. The north magnetic pole 
is at some distance from the axial North Pole. The 
magnetic pole of the earth nearest the North Pole is 
termed the North Magnetic Pole, Fig. 5. A compass needle 

points its north-seek- 
NORTH MAGNETIC/ \^ X ing polc in the direc- 

tion of the north 

^EQUATOR 

magnetic pole. This 
is in reality a south 
pole; but as it seeks 
the north pole, we des- 
ignate it as the north 
pole. This deviation 
of the magnetic pole 
from the axial pole 
becomes more and 
more marked as the 




SOUTH POLE 



Fig. 5. — North Magnetic Pole. 



compass needle is located farther and farther north on 
the earth. As the compass needle approaches the north 



4 EXPERIMENTAL ELECTRICITY 

geographical pole of the earth, it is affected by the north 
magnetic pole in the same manner as it would be if it were 
approaching the pole of a magnet; thus it dips more and 
more. The north magnetic pole in the northern hemi- 
sphere of the earth is located at Boothia Felix, Fig. 6, at 
about latitude 70 and longitude 96 west. 



/" ' '^/^NvC/ ^ / ^i^T^^^^^ShC\3^^^ \ X^\ "^ 


/xK € / 

'V 7 ^ /^■^X4— , 

\i/o: 130 11/1^0= ^-^i*^ . "f 


— L \ Bay \\ 4A \/(iX^ 



Fig. 6. — Magnetic Pole. 

Horseshoe Magnet. — If a bar of steel in the form of a 
horseshoe be magnetized, it will be found that the strength 
of the magnet appears to be greater than if the same piece 
of steel had been magnetized in the shape of a bar. This 
is due to the fact that the magnetic emanations which issue 
from the north pole cover an air gap, as indicated by the 
curved lines in Fig. i, and enter the south pole. In travers- 
ing this air gap the magnetic emanations suffer a much 
greater resistance, termed rehictance for magnetic circuits, 
than if iron were substituted for it. In bringing the north 



MAGNETISM 




Fig. 7. — Horseshoe Magnet from Weston Voltmeter. 



6 EXPERIMENTAL ELECTRICITY 

and south poles close together as in a horseshoe magnet, 
the air gap, or path of the magnetic circuit, is considerably 
reduced, increasing the apparent strength of the magnet. 
Figure 7 illustrates an excellent and powerful horsesho6 
magnet used by the Weston Instrument Company for their 
portable ammeters and voltmeters for direct current work. 
By shaping the pole pieces of a magnet in the form of arcs, 
Fig. 22, it is possible to distribute the magnetic emanations 
and accommodate a movable coil and iron core, reducing the 
magnetic leakage to a minimum. The presence of a circu- 
lar iron core reducing the air gap to a minimum, Fig. 22, 
seems to facilitate the passage of these magnetic emanations. 
We speak of such magnetic emanations as lines of force ^ 
and of the total lines of force as the flux of a magnet. 
Where the flux per square inch or per square centimeter 
is meant, the term flux density is employed. Most mag- 
netic formulas employ the unit flux density, and in cal- 
culating the total flux, care must be taken to multiply by 
the area. 

Lines of force pass much more readily i^t iron than in 
air. 

Molecular Theory of Magnetism. — Ewing carried on a 
large number of tests which seemed to indicate that a 

I — —7:; r-q piece of unmagnetized iron or steel is 

a composed of a large number of infinitely 

small magnets that assume geometric 

^.—.=z=^=.—.=zs I figQj-es^ pig_ 8 a. When a magnetizing 

Fig. 8. — Molecular forcc is placed in the vicinity of a piece 
Magnets. q£ steel SO that the steel becomes magne- 

tized, all of these molecular magnets arrange themselves 
in parallel formation as in Fig. 8 b. This increases the 
magnetic strength of the steel, producing a north pole 
at one end and a south pole at the other end. 



MAGNETISM 



Experiment 3. Take a sewing needle, unmagnetized, and dip it in 
iron filings ; no filings will cling to it. Magnetize the needle by strok- 
ing it with one pole of a bar magnet, dip it in filings again, and notice 
that the filings are attracted to the ends of 
the needle. Break the magnetized needle in 
two, dip both magnetized pieces in filings. 




Fig. 9. — Magnets made 
by breaking Magnetized 
Needle. 



duced. Break one of the smaller pieces in 

two, and dip the pieces in fihngs, and notice 

that two more magnets are produced. This 

process of breaking the smaller pieces in two 

could be continued until only a molecule of the material remained, when 

it would probably be found that the molecule was a magnet possessing 

a north and a south pole. (This experiment can be readily performed 

on a vertical lantern.) 

Magnetic Induction. — A piece of soft iron placed in the 
vicinity of a magnet or in contact with the magnet assumes 
the properties of the magnet, attracting iron filings, Fig. 10. 
This phenomenon is termed magnetic induction. The ex- 
planation of this phenomenon is that the soft iron placed 
in the magnetic field has its mole- 
cules so arranged that the soft iron 
becomes a magnet. It is necessary 
to have the soft iron in proximity to 
the magnet in order that its influ- 
ence may be felt, for if the magnet 
be removed from the soft iron, the 
filings will fall from the end of the 
soft iron bar. When the magnet has 
been completely removed, a small 
amount of magnetism remains in 
the bar. This effect is designated 
by the term retentivity. It is a form of molecular inertia. 
When the molecules of a piece of steel or soft iron are 
under the process of magnetization, the molecules re- 



Ji 



™,}f 



Fig. 10. — Magnetic Induc- 
tion, 



8 



EXPERIMENTAL ELECTRICITY 



arrange themselves as described in the paragraph explain- 
ing the molecular theory of magnetism. This arrangement 
of the molecules becomes fixed with a piece of steel, whereas 
the effect is temporary with soft iron. When the steel or 
soft iron is removed from the influence of the magnetizing 
medium, there is a tendency for the new molecular ar- 
rangement to remain. It requires a certain amount of 
magnetizing in the opposite direction to reduce to zero the 
magnetism of a piece of iron once under the influence of 
a magnetizing force. A more complete exposition of the 
retentivity of soft iron is given later in treating of mag- 
netization and hysteresis curves. 

Experiment 4. Place a piece of soft iron in iron filings and notice 
that no filings are attracted to it. Place one pole of a magnet in con- 
tact with the iron bar, dip one end of the bar in filings, and notice that 
filings adhere to it, Fig. 10. Remove the magnet from contact with 
the iron bar, and notice that, while most of the iron filings fall immedi- 
ately, there are still a few filings that cling to the soft iron. 

Effect of Temperature upon 

aMagnetizability. — Temper a- 
ture plays a very important 
part in the question of mag- 
netizability. A magnet when 
heated to a dull red heat loses 
its magnetism to a marked 
degree. A small piece of iron 
when at red heat will not be 
attracted by a magnet. It is 
natural to suppose that if the 
molecular theory of magnetism 
be true, then when a magnet is 
,. . T .^ heated and its molecules are 

Fig. II. — Magnetization Loss with 

Heat. set in vibration, they would 



s> 



MAGNETISM 



9 



tend to lose their fixed position due to their magnetiza- 
tion. 

Experiment 5. Mount a small wire nail upon a piece of platinum 
wire and suspend it from a support in a clip stand, Fig. 1 1 . Heat the 
nail to redness by a Bunsen burner, and then bring a magnet into 
proximity to the nail, noting that the nail is not attracted to the mag- 
net. Watch the nail cool, and notice that, as its temperature lowers, 
owing to the absence of the Bunsen burner, a critical temperature is 
reached where the nail is suddenly attracted to the magnet. 

The Effect of Vibration upon Magnetizability. — Care 

should be taken in handling magnets not to allow them to 
suffer shocks of any kind, such as falling, since they are 
likely to suffer loss of magnetism from such shocks. This 
can be explained according to the molecular 
theory of magnetism on the assumption that 
the molecules are not permanently fixed in 
their new positions, and that vibration tends to 
allow them to return to their former positions. 
A small piece of iron, termed a keeper, Fig. 12, 
is commonly used to connect the north and 
the south poles of a magnet, forming a closed 
magnetic circuit. When using the magnet, 
the keeper should be pulled quickly from the 
magnet ; it should not be allowed to snap back, 
striking the poles, but should be allowed to 
make the contact slowly. By carefully fol- 
lowing these directions, it is possible, after a large number 
of trials, to increase the magnetism of a magnet quite per- 
ceptibly. 

The Obtaining of a Hysteresis Cycle. — When a piece of 
soft iron is magnetized in one direction and then the 
magnetizing source removed, it is found that the soft iron 
does not lose its magnetism entirely^ but retains part of it. 




Fig. 12. — 

Keeper and 

Magnet. 



lO EXPERIMENTAL ELECTRICITY 

As the iron is hardened or as it contains a greater propor- 
tion of carbon, it is found that when subjected to the same 
magnetizing force as before and the magnetizing force is 
removed, it retains a greater amount of this magnetism, 
becoming more and more of a permanent magnet. When 
steel is used, it retains considerable of its magnetism, be- 
coming indeed a permanent magnet. In order to abstract 
this remaining magnetism from soft iron, or to reduce its 
magnetism to zero, a certain amount of energy must be 
used in demagnetizing it. This loss of energy, due to the 
retentivity of the iron or due to its molecular inertia, is 
termed hysteresis loss. It is not necessary to reduce the 
magnetism to zero in a sample of iron to have a hysteresis 
loss, but it is simply necessary first to increase its mag- 
netic strength, then to lower, and then to bring it back to 
its original state. (See curves. Fig. 15.) Where soft iron 
is used and its magnetic condition is being continually 
changed, that is, when it is first magnetized in one direc- 
tion, then in the other, the hysteresis loss may become 
quite a factor. In an alternating current transformer such 
a condition exists. With the modern methods of manu- 
facture of transformer iron, involving the working of the 
iron through certain temperature ranges, and with the 
addition of certain ingredients to the iron, such as tung- 
sten, the hysteresis loss has been reduced to a marked 
extent, and the permeability of the iron has been increased, 
reducing the weight of iron necessary for a given per- 
formance in a given piece of apparatus. 

A curve which shows the magnetic changes in a sample 
of iron when carried through a cycle of magnetization is 
termed a hysteresis curve. 

Experiment 6. Take an iron ring about 5 inches in diameter and 
I inch by 2 inches in thickness, and wind upon it about 100 turns of 



MAGNETISM 



II 



REVERSING SWITCH 




+ A.t 



GALVANOMETER 



Fig. 13. 



Set up for obtaining Hysteresis 
Curve. 



No. 16 insulated wire, Fig. 13. Over this coil wind about 2 turns of wire 
and connect the wire to a galvanometer. It may be necessary to de- 
crease this secondary coil to 
one turn or to increase it to 
5 turns according to the sen- 
sitiveness of the galvanom- 
eter, the quality of the iron 
used, or the maximum deflec- 
tion obtained. To the pri- 
mary coil of a large number 
of turns connect a reversing 
switch, through an ammeter 

and a lamp board, to a i i6-volt service. The lamp board should contain 
two 50-candle-power lamps and three i6-candle-power lamps, which can 
be connected in parallel. Now close the commutating switch in one direc- 
tion and then turn on all of the lamps slowly, starting with the smaller 
units. When ready to take observations, first turn off one 50-candle- 
power lamp, noting the deflection or kick of the galvanometer needle. 
Turn off a second 50-candle-power lamp, then a i6-candle-power lamp, 
a second 16, and then a third i6-candle-power lamp, in each case noting 
the direction and magnitude of the galvanometer deflection. Then throw 
the reversing switch, and turn on one i6-candle-power lamp, then the 
second i6-can die-power lamp, then the third i6-candle-power lamp, then 
a 50-candle-power lamp, and then a second 50-candle-povver lamp. In 
each case note the deflection and the direction of the needle. Some of 
the readings will be small, while others will be quite large. The iron is 
now magnetized to a maximum value in the opposite direction to that 
from which it was magnetized when the experiment was started. To 
come back to the original starting magnetic condition, turn off" one 50- 
candle-power lamp, then a second 50-candle-power lamp, then a i6-candle- 
power, a second i6-candle-power, and a third i6-candle-power lamp. 
Throw the reversing switch. Turn on one i6-candle-power lamp, a 
second i6-candle-power, a third i6-candle-power, a 50-candle-power, and 
a second 50-candle-power lamp. The iron has now been returned to its 
original state of magnetization. In coming back, all of the deflections 
will be in the opposite direction. (If a mistake is made in throwing the 
commutating switch at the wrong time, or in turning the lamps on and 
off at the wrong time, the experiment must be done over with the proper 
sequence. Be sure to have lamp sockets making a clean break.) 



12 



EXPERIMENTAL ELECTRICITY 



How to plot the Hysteresis Curve. — With the previous set of ob- 
servations obtain a sheet of coordinate paper ^ and divide it into four 
quadrants. The horizontal axis, or abscissa, is divided up into equal 

sections, representing usually am- 
pere turns, but in this case repre- 
senting i6-candle-power equiva- 
lents. A 50-candle-power lamp 
can be taken as equal to three 
1 6-can die-power equivalents. 
Positions i, 2, 3, 6, 9, to the right 
and to the left of the vertical 
zero axis indicate the respective 
ampere equivalents. Readings 
to the right of the central verti- 
cal line, zero or ordinate, indi- 
cate positive values, and readings 
to the left indicate negative 
values. The first current value with all of the lamps turned on is line 
9 to the right, or A. All of the deflections of the galvanometer 
should then be added up, that is, all positive values and all negative 
values going in one direction. These totals should be nearly equal. 
Theoretically they should be precisely equal. The readings may be 
tabulated for convenience as in the accompanying table, where the total 
is 30. 









" 






1 


1 


































1 












[ 




A 


















1 








^ 


^ 




s* 


















1 


2 


^ 


,1 


4 






















\\ 


^ 


/ 


< 




C 
























4J/ 




/ 






P 








M/ 


GN 


ETl 


^IN 


3 F 


3R( 


E 


'H' 


■/ 


/ 








lU 








- 


- 


- 


- 


"~ 


- 


- 


5 

7 


U^ 


f 

r 


-- 


- 


- 


.-_ 


- 




- 














fi 


/ 

































y 
































h 


Y 


/ 


^1 ' 














s 


r 






4 




-^ 


/' 


.^ 




[co 


ERC 


IVI 


rv 








;; 


2 
B 




SS! 


^ 






^ 


r 


I 














H 


















1 














§ 




















1 














2 





; Z 'J I 1 '2 6 6 

Fig. 14. Hysteresis Curve. 



Current Equivalents 


+ Readings 


I 50 = 3 


16 


I 


I 50 = 3 


16 


2 


I 


16 


2 


I 


16 


I 


I 


16 


4 

COMMUTATE 


I 


16 


5 


I 


16 


6 


I 


16 


3 


I 50= 3 


16 


4 


I 50 = 3 


16 


2 



— Readings 
I 
2 
2 

I 

4 

COMMUTATK 

5 
6 

3 

4 
2 



30 



30 



MAGNETISM 



13 



Fifteen of these values are plotted above the central zero horizontal line 
and 15 below. A point should then be located where the horizontal 
line 15, above o abscissa, intersects with the hne A^ Fig. 14. This 
point is the starting point of the curve. Where line 6 intersects with 



B 
















_^_ 




— 




' / 


y 










^ 


^ 












/ 


/ 








y 


^ 














/ 


/ 


' 






y 


/ 














/ 


/ 








6000 
















/ 


/ 


















^^ 




p^ 


1 












5000 






^ 




^ 


^ 


/ 


/ 














y. 




-^ 








/ 














4000 








-^ 


-^ 


^ 


7 
















,^ 




'^ 






/ 


















«4- 








/ 


/ 






















^ 


K 


1 


















200' 


^ 






/ 




























/ 




INCO 


VIPLETE 
DF ANr 


HYS 
lEALE 


■ERES 
3 IRO^ 


S CYC 


ES 










^^ 


^^ 


f 






















^:^ 




/ 

























^ 


^ 

























H -3 1.00 1.5 2.0 2.5 3.00 

Fig. 15. — Partial Hysteresis Curves. 

line 14, the second point should be located. This is obtained by sub- 
tracting deflection i from 15. The second point is at the intersection 
of line 3 with line 12, etc. The process of subtraction should be con- 
tinued for the ordinates until the zero line is reached, and then the read- 
ings should be added. When one half of the curve has been plotted, 
the other half should be plotted in the reverse manner, starting to sub- 



14 



EXPERIMENTAL ELECTRICITY 



tract the deflections from the point B. In an actual test, where quanti- 
tative results are to be obtained, an ammeter is inserted in series with 
the lamp board or some other adjustable resistance, the length of the 
magnetic circuit is obtained, as well as the amperes, the constant of the 
galvanometer is determined, and the 2s:\\x2\ flux density for each mag- 
netizmg force is calculated.* 

Theory of Experiment. — Every time the current 
strength in the primary coil is changed, the magnetic 
flux in the iron changes by a certain amount. This 
change of flux, cutting the secondary coil, induces an elec- 
tro-motive force in the coil according to 
the laws of electro-magnetic induction 
(see page 46). The electro-motive force 
generated sends a current through the 
galvanometer directly proportional to 
the change in flux, causing a propor- 
tional throw of the needle. 

Magnetic Field. — The space around 
a magnet where its influence is felt is 
termed a magnetic field. A compass 
needle moved at any point in this field 
sets itself parallel to the Hues of force 
of the field. 






'jt 



Fig. 16. — Magnetic 
Spectrum. 



Fig. 17. — Shaker. 



vA 



eee 



Fig. 18.— Path of 
Lines of Force. 



W-; 



©©e 



'"-m 



Fig. 19. — Cross Mag- 
netization. 



Experiment 7. — Place on the condensers of 
a vertical lantern arranged for projection on a 
screen, or for laboratory work on a table, a small 
bar magnet. Cover this magnet with a sheet of 
glass and sprinkle filings over it, tap the glass, 
and obtain a magnetic spectrum, Fig. i. Cover 
the spectrum with an additional piece of glass. 
In spreading the filings, a small shaker. Fig. 17, 
made with a fine-mesh wire netting over the bot- 



* For formulae see Thomson's Elementary Lessons in Electricity, pages 354- 
363 ; Foster's Handbook, pages 89-93. 



MAGNETISM 



15 



torn of a hole bored in wood will serve to good advantage. If the glass 
is tapped with a pencil after the filings have been spread upon it, the 
spectrum will assume the proper shape. 

When the second piece of glass is in position, place a small com- 
pass needle over the glass as in Fig. 16, and move it to different points 
in the magnetic field. The compass needle will set itself parallel to 
the lines of force. 

Magnets which come into contact with other magnets 
are likely to have consequent poles. These and other 
magnets, with holes in them, made from the blades of 
Gillette's safety razors, Fig. 18, show interesting magnetic 
spectrums. It is even possible to magnetize a small safety 
razor blade so that it will have two north poles, Fig. 19. 

A Magnet Pole travels along Lines of Force. — The fol- 
lowing experiment demonstrates the fact that a magnet pole 
free to travel will follow in its path the lines of force of a 
magnetic field. 



MAGNET POLE 



GLASS DISH 




-ELECTRO-MAGNET 



^^ 



CONDENSER 



REVERSING^ 
SWITCH 



Experiment 8. — Connect a small electro-magnet through a reversing 
switch and a i6-can die-power lamp in series with a i i6-volt direct current 
circuit, Fig. 2.o,.a. Over the magnet, Fig. 20^, place a glass plate sup- 
porting a shallow dish contain- 
ing water to a depth of \ of an 
inch, 20 a. The glass dish 
should be as close to the magnet 
as possible. Through a small 
piece of cork about \ of an inch 
square pass a small piece of 
needle which has been mag- 
netized. Float the small magnet 
upon the water so that it will be 
in an upright position with the 
north pole submerged in the 
water. The lower pole will be 
attracted or repelled by whatever 
pole of the electro-magnet it happens to be near. It is well, before 
placing the dish in position over the magnet, to cover the electro-magnet 




16 VOLTS 
d.C. 



Fig. 20 a. — Magnet Pole travels along 
Lines of Force. 




1 6 EXPERIMENTAL ELECTRICITY 

with a piece of glass and obtain a magnetic spectrum with iron filings so 
that it can be shown that the magnet pole will travel along the path of 
the magnetic spectrum. Place the current on the circuit, and the mag- 
net pole will be attracted to one pole of the electro-magnet. Throw 
the reversing switch, and the magnet pole will 
be repelled, traveling along one of the lines 
of the magnetic spectrum, being attracted to 
the other pole. When the magnetic pole is 
approaching the pole of the electro-magnet, if 
the switch is thrown, the magnet pole will con- 
FiG. 20 /^. — Small Elec- tinue its motion past the pole, circling around 
tro-magnet. -j-q j^g original starting place. It requires a little 

practice to throw the reversing switch at the right time. It is not well 
to let the magnet pole get too near the edge of the dish, since it will be 
attracted to the side of the dish by capillary attraction. If it looks as 
though this was likely to occur, throw the reversing switch, attract- 
ing the magnet back to its original pole, and then throw it again, 
etc. This is a lantern experiment. 

Mayer's Needles. — The following experiment serves to 
illustrate the molecular theory of magnetism in showing 
the relation of the mole- 
cules of a sample of iron / \^ 

before magnetization oc- t I 

curs. Small magnets 

made from needles, as in fig. 21. - Molecular Ma^ 

Experiment 3, are floated 

horizontally in a dish of water, so that their poles can be 
attracted to each other. After a certain time the magnets 
will arrange themselves in geometric figures, Fig. 21, de- 
pending upon the number present. If difficulty is experi- 
enced in preventing the magnets from being attracted to 
the side of the vessel by capillary attraction, a single pole 
of a bar magnet held over the dish or under it will serve 
to keep the magnets from being attracted to the side of 
the dish. 



MAGNETISM 



17 



Experiment 9. Perform the experiment described, using an-angement 
of tank as in Experiment 8. 

Practical Application of Permanent Magnets. — Perma- 
nent magnets are used extensively in portable and station 
types of ammeters 
and voltmeters sim- 
ilar to the Weston 
type, Figs. 22, 23, 
24, 25, 26, as a 
means of produc- 
ing load in Thom- 
son Recording 
Wattmeters, Figs. 
27, 28. They may 
also be found in 
nearly all tele- "-^ 

Dhone receivers ^'^*^' 22. — Movement of Weston Standard Voltmeter. 

Fig. 29. It is quite important that magnets used for these 
purposes should not change appreciably In 






Fig. 23. —Weston Standard '^Tolt- 
meter Complete. 



Fig. 24. — Station Type of 
Weston Voltmeter. 



strength with time. They should not only have high mag- 
netic strength, but they should not lose this strength when 
used. To accomplish this, they are put through a process 



i8 



EXPERIMENTAL ELECTRICITY 



of aging. They are subjected to certain temperature 
changes and to certain vibrations, so as to shake off all 
*' loose magnetism." Permanent magnets are also used in 




25. — Laboratory Standard 
Weston Voltmeter. 




Fig. 26. — Weston Station 
T}'pe of Ammeter. 



Weston Speed Tachometers and in relays, Figs. 30, 31. In 
the recording wattmeter a copper disc, Fig. 32, revolves be- 
tween the poles of a magnet, generating eddy citiTents in 
the disc. The disc is equivalent to a loop of wire short-cir- 
cuited upon itself, rotating in a magnetic field, generating an 
e. m. f. in the disc. This e. m. f. causes a current to circulate 
through the disc, and the current causes a counter torque or 
resistance to rotation, due to its magnetic field. This effect 
varies directly zuith the speed of the meter disc or produces a 

load vv^hich varies with the speed 
of the meter. This matter is dis- 
cussed at greater length under 
recording wattmeters. By mov- 
ing the magnets, Fig. 28, in or 
out so that a greater or a 
smaller radius for the magnetic 
pull may be obtained, the speed of the meter may be 
varied as much as 15% for a given load. This adjustment 
constitutes the full load adjustment of the meter. 




Fig. 27. — M.ignets of Thomson 
Recording Wattmeter. 



MAGNETISM 



19 



Ammeters and Voltmeters. — Many types of instruments 
have been developed in the past for the measurement of 
current and electromotive force. Such instruments are 
termed ammeters and voltmeters. They utilized certain 
effects of the current, such as the attraction of a solenoid 
for a core of iron and the effect of temperature. Most of 
these instruments, however, as first constructed, were large 
and cumbersome ; they were difficult to repair, were af- 
fected by temperature changes, were more or less inaccu- 



Brush 



Lc 



LS't -> 




Fig. 28. — Thomson Recording Wattmeter. 

rate, were not dead-beat, and required some time before 
the needle, or pointer, would come to rest. The develop- 
ment of the D'Arsonval type of instrument with permanent 
magnets and movable coil marked a great step in advance 
in instrument making. Dr. Edward Weston first developed 
the standard portable instrument of this type, now in com- 



20 



EXPERIMENTAL ELECTRICITY 




mon use, and this has greatly contributed toward electrical 
progress. Weston ammeters and voltmeters are provided 
with scales with uniform graduations ; they are dead-beat ; 
they are small and extremely accurate; they 
can be easily transported, repaired, and 
standardized ; and they have been awarded 
prizes at expositions all over the world. 

The fundamental principle of both the am- 
meter and the voltmeter is the same. The 
instrument, Fig. 22, consists of a powerful 
horseshoe magnet having curved pole pieces 
so as to accommodate a circular iron core and 
a movable coil. The magnets are carefully 
made of the best selected steel, so that their 
Fig. 29, — Tele- magnetism will be strong and permanent, 
phone Receiver Herein lics a most important feature of the 
^ • • ■ ^^' instrument. A movable coil wound upon a 
metal shell is pivoted so that it will move freely over the 
circular iron core without touching the pole pieces. The 
air gaps are 
made as small 
as possible, so 
as just to per- 
mit clearance. 
The small air 
gap serves to 
increase the 
strength of the 
magnetic cir- 
cuit. Spiral 
springs are 
mounted on 
both sides of the movable coil, top and bottom, the inner 




Fig. 30. — Weston Speed Tachometers. 





MAGNETISM 21 

ends of the spirals being connected to the terminals of the 
movable coil in order to form one complete circuit, throu,e:h 
which passes the electric current when the 
instrument is in operation. Upon the top of 
the movable coil is mounted a pointer consist- 
ing of a light aluminium tube which is flat- 
tened where it moves over the instrument 
scale, and balanced upon 
the other end with a fig. 31.— v^^eston 
counterpoise weight. ^^'^y- 

The whole moving element is very 
light, having practically no inertia. 

FIG. 32. -Wattmeter Disc, ^yj^^^ ^j^^ pointer is mOVCd tO OUC 

side and released, it returns to zero by means of the spiral 
springs which are fastened at their other extremities to 
movable supports. When the electric current enters and 
passes through the movable coil, it magnetizes this, 
producing poles which are repulsed by the poles of the 
permanent magnet, thus causing a deflection. As the 
permanent magnets have a constant field strength, and 
as the spiral springs exert a counter torque propor- 
tional to the amount of twist, the deflection of the 
needle is proportional to the current passing through the 
movable coil. The small metal shell upon which the mov- 
able coil is wound has a current circulating through it as 
it moves through the magnetic field of the permanent mag- 
nets. This current is due to an induced e.m.f. and opposes 
the viotion of the coil, not the deflection. As this torque 
is only effective when the coil is in motion, the movable 
coil comes quickly to rest without any swinging of the 
needle. The needle will start from zero, and rise quickly 
as it deflects, following such changes in the current as 
occur with an entire absence of swinging. The amount of 



22 



EXPERIMENTAL ELECTRICITY 



current necessary to produce full scale deflection when 
passed through the movable coil is quite small. With a 
150-volt scale with 15,000 ohms in the circuit, a current of 
^^ of an ampere flows. If the instrument is to be used 

as a voltmeter, a high 
resistance of about 100 
ohms to the volt is 
placed in series with the 
binding posts leading 
from the movable coil. 
Thus a 3-volt scale 
would have about 340 
ohms, and a 150-volt 
scale would have 16,000 
ohms. The higher this 
resistance, the less the 
instrument will disturb 
the circuit when con- 
nected to it. Where the 
instrument is used as an 
ammeter, the movable coil is shunted by a low resist- 
ance. Fig. 33, which is placed in series with the load. 
This shunt may be internally connected, as in Fig. 26, or 
externally connected, as in Fig. 33. The form of ammeter 
shown in Fig. 33, with a central zero, is especially conven- 
ient for laboratory work, as it may be used without the 
shunt as a galvanometer. The ammeter is in reality a low 
range voltmeter or milli-voltmeter measuring the drop in 
potential across the shunt. A small mirror is mounted 
under the pointer of the instrument, so that when taking 
observations the operator looks down in the mirror and sets 
the reflection of the needle of the instrument on a Hne with 
the needle itself, thus eliminating parallax. 




Fig. 33. 



■Weston Ammeter, Central Zero 
with Shunt. 



MAGNETISM 23 

The scale may be so arranged as to indicate two sets of 
values, such as 150 volts and 300 volts. For details as to 
the calibration of voltmeter and ammeters, see chapter on 
measurements. 

QUESTIONS 

1. Give a complete explanation of the molecular theory of mag- 
netism. 

2. How would the shapes of two hysteresis curves compare, one for 
a good value of soft iron, the other for annealed steel? 

3. What advantage is there in using for a generator, iron with a high 
permeability ? 

4. How would you convert the flux density expressed in lines of 
force per square inch to lines of force per square centimeter ? 

5. In what direction would the north pole of a compass point if the 
observer were located at the north geographical pole ? 

6. On the assumption that magnetism cannot be insulated, how do 
you explain the action of the laminated sheet-iron shields, or inclosing 
cases, used around watches ? 

7. In demagnetizing a watch by placing it for a time in a coil carry- 
ing alternating current, it is often necessary to repeat the operation 
several times. Upon what part of the hysteresis cycle of the steel of 
the hairspring must the watch be taken from the coil in order that the 
magnetism must be zero ? 

8. Approximately how many times stronger would the ampere turns 
of the field and armature have to be if all of the iron were taken from 
an electrical generator ? Would the electrical industry exist if it were 
not for the magnetic properties of iron ? 

9. Assuming a heavy short circuit to occur with a Thomson record- 
ing wattmeter in circuit, one which happened to be of such magnitude 
as to demagnetize the magnets, how would the short circuit affect the 
accuracy of the meter ? 

10. Suppose that in course of time the magnets of a voltmeter should 
become weakened, how would it affect the accuracy of the meter ? 

11. What relation does a magnetization curve bear to a hysteresis 
curve, and how would you plot them both together for the same sample 
of iron ? 

12. Why should the presence of iron be avoided as far as possible 
when using compasses or the magnet type of voltmeter and ammeter ? 



24 EXPERIMENTAL ELECTRICITY 

13. How would you proceed to magnetize a piece of steel with a bar 
magnet, if you wished to obtain a given pole at a particular end of the 
piece ? 

14. Give a few examples of the use of permanent magnets not men- 
tioned in the text. 

15. In relays why is it sometimes necessary to place a piece of 
paper over the poles of the magnets to prevent the armature from stick- 
ing fast ? 

16. Why does steel make better magnets than wrought iron ? 

17. Why does soft iron have a smaller hysteresis loss than good 
steel ? 

18. In using a mariner's compass, why is it necessary to correct the 
observation in order to determine the true direction of the motion of a 
vessel ? 

19. Compare the earth to a magnet, showing how a compass would 
act when placed at various locations on its surface. 



CHAPTER II 



ELECTRO-MAGNETISM 



THE MAGNETIC FIELD AROUND A STRAIGHT WIRE 
CARRYING A CURRENT 

Experiment lo. Bore a small hole through a glass plate so that a 
No. 1 6 copper wire will pass through it, and mount the glass upon a 
support from which it may be quickly removed. Send a current of 
about 5 amperes through a wire inserted through the hole in the plate. 
Sprinkle the plate with filings and tap the plate. Remove the wire 
from plate, and note the magnetic spec- 
trum, Fig. 34. This spectrum may be 
projected on a screen by placing the glass 
plate in the slide carrier on a vertical lan- 
tern. 



5 AMPERES 




Fig. 34.— Magnetic Field 
around Wire. 



The experiment just described 
proves that there is a magnetic 
field around a conductor carrying a 
current. This magnetic field is similar in all respects to 
the magnetic field produced by a bar magnet. This fact 
may be further demonstrated in the following way : 



^ " ^^ 



Ul — — 1 Experiment 11. Send a current of 5 amperes 

^ through a straight wire, and dip the wire in iron 

filings. The fihngs will cling to the wire, as in 

Fig- 35- 

Experiment 12. Send a current of 5 amperes 
through a straight con- 
ductor, and place the wire 

first over a compass and then under a compass. 

Notice that the needle will deflect in both cases 

at right angles to the wire, and that it will 



Fig. 35. — Presence of 
Field shown with 
Filings. 



h 



Fig. 36. — Field shown 
with Compass. 

point in opposite directions when above and below the wire. Fig. 36. 

25 




26 EXPERIMENTAL ELECTRICITY 

The Compass Needle always tends to set itself Parallel 
to the Lines of Force. — To determine the direction in 
which the needle will point, it is well to assume that one 
is looking along the wire from the positive end, and that 

the lines of force are circling 
around the wire in whirls in a 
clockwise direction. The positive 
pole of the compass will point 
FIG. 37. -Support for Compass -^^ ^j^g direction of the whirl, as 

for Projection. . . 

if It were lollowmg this around. 
For instance, if the current flows from a positive terminal 
from north to south, the compass, when placed over the 
wire, will point west. The letter combination S-N-O-W 
affords a convenient way of remembering the direction. 
Current passing from South to the Noi^th ( + ), Over the 
compass, needle will point West. Experiment 12 may be 
conveniently performed on a horizontal or a vertical lan- 
tern by mounting the compass needle in a wooden support 
with a hole in it, Fig. 37, and supporting it in a clip stand. 
Where large cables are carrying heavy currents, such as 
a single 1,000,000 circular mil cable carrying 1000 amperes, 
the magnetic field surrounding the cable is very strong. 
Where such a magnetic field affects the accuracy of am- 
meters, voltmeters, etc., it is termed a stray field. Stray 
fields are very strong on switchboards where the hcsses be- 
hind the board carry large currents. It is sometimes nec- 
essary to install astatic instruments to eliminate the effects 
of stray fields. 



+ 5 AMPERES 

116 -' I ^ -^ 

VOLTS .= •^^y ^ I -J^ 



Experiment 13. i. Bend a wire back upon ^ 

itself, as in Fig. 38, and send 5 amperes through ^'^^- f'^ " Neutralizaiion 

' t> J ? J J- & Qf Magnetic Field. 

It. Bring this double wire into the vicinity of 

a compass, and notice that the compass needle is not deflected. The 

current passing in opposite directions through the wire neutralizes the 



ELECTRO-MA GNETISM 



27 



^ 



magnetic field. This experiment may be performed with the lantern. 
A small compass may be used to test the presence of current in a cable 
where cables consist of one conductor , but where the cables are con- 
centric, two in one sheath, the test is not so satis- 
factory. 2. Bring a flexible wire carrying a current 
into the vicinity of a magnet, preferably an electro- 
magnet. The wire should carry about ten amperes, 
and may be one of the leads of an electro-magnet. 
A flexible silk-covered wire is most suitable for this v 
experiment. If the wire is brought near the mag- +" 
net, slightly over it, and lowered quickly, it will en- Z~ 

. ,1 „. 116 Vui.10 

Circle the magnet. Fig. 39. d.c. 

Fig. 39. — Wire carry- 

The foregoing experiment illustrates ing Current tends to 

cncirclG IVIs-sriGt 

the fact that the magnetic field around a 
straight wire tends to adjust itself so as to assist the mag- 
netic field of the electro -magnet, or so that its lines of 
force will be parallel to those of the electro-magnet. 

Effect of Current in a Coiled Wire. — A coil of wire con- 
sisting of a number of turns of 
equal diameter is termed a helix. 
By coiling up a length of wire 
in this manner, Fig. 40, and 
sending a current of electricity 
through it, it is possible to con- 
centrate nearly all of the lines of force of all the coils so 
that they will have a central path through the helix. 




Fig. 40. — Helix. 



Experiment 14. Bring near the opening of a helix a compass needle, 
and notice the poles at each end, Fig 40. 



A Large Coil for Experimental Purposes (Long Tom). — 
The coil, Fig. 41, about to be described is very useful for 
all forms of magnetic experiments, and when supplied 
with an iron core becomes a strong electro -magnet. It is 
also useful as a variable inductance for alternating current 



28 



EXPERIMENTAL ELECTRICITY 







experiments. Such a coil, popularly known to the author's 
students as "■ Long Tom," may here be distinguished by 
that name for convenience. This coil is about 12 inches 

-g g high and 5 inches in diameter. A hole 

^/^0^ // through the center has a diameter of slightly 

over \\ inches, so as to admit a i^-inch iron 

rod. In order to secure its rigidity the coil 

is wound upon a split brass shell with 3000 

turns of No. 16 wire. It has a resistance 

of about 10 ohms. When connected for a 

Fig 41 —Solenoid short time to a I i6-volt direct current circuit, 

(termed Long and whcn Supplied with an iron core, this 

^°™^- coil becomes a powerful electro-magnet. It 

is provided with a heavy slate upper and lower base and 

with double binding posts. 

Experiment 15. Mount a coil asjust described over a support 12 inches 
from the floor, so that a i^-inch iron rod resting on the floor will project 
up into the coil about 4 inches, as in 
Fig. 42. Open and close a circuit of 
116 volts through this coil, and the 
iron core will be drawn up in the coil, 
and will then fall, striking the floor 
when the circuit is broken, thus pro- 
ducing a trip hammer effect. When 
the core is suspended in the coil, catch 
hold of the core and lift the coil from 
its supports, thus showing the great 
strength of the magnet. By opening 
and closing the circuit at the right in- ^^^- 42- -Sucking Coil. 

tervals, it is sometimes possible to make the core jump out of the coil. 

When an iron core is drawn into a solenoid, as in the 
previous experiment, it is due to the fact that the core be- 
comes magnetized by induction, and also because iron in 
a magnetic circuit always adjusts itself so as to accommo- 




ELECTRO-MA GNETISM 



29 




Fig. 43. 



Limit Switch. (West- 
inghouse.) 



date the greatest number of lines of force. With a given 
magnetization 10,000 times as many Hnes of force may 
pass through a circuit of modern iron than if air alone 
were present. The principle of the solenoid's attracting an 
iron core is widely used in engi- 
neering. In railway operation, 
for instance, devices termed 
limit switches. Fig 43, consist- 
ing of a movable core actuated 
by a solenoid, are used to regu- 
late the input of current into 
railway motors opening and 
closing switches which short 
circuit resistance. This method 
is also used on protective de- 
vices such as circuit breakers, 
relays, oil switches, etc. Relays are termed straight-over- 
load or time-limit relays. With some types of time-limit 
relay, the action of the plunger of a solenoid is resisted by 
a small air bellows with a shght opening in it. The time 
taken for the plunger to force the air out in its travel is the 
time element in the circuit. When the plunger has finished 
its travel, it closes contacts which in connection with auxil- 
iary apparatus, such as a 120- volt storage battery and the 
motor or solenoid on the oil switch, open the circuit. The 
greater the overload current, the stronger will be the at- 
traction of the coil for the plunger and the shorter will be 
the time necessary to open the circuit. Relays equipped 
with bellows which operate in the manner described are 
termed inverse time-limit relays. 

The Electro-dynamometer. — This instrument was em- 
ployed for many years to measure alternating currents 
before the portable instruments now in use were developed. 



30 



EXPERIMENTAL ELECTRICITY 



The electro-dynamometer, Fig. 44, consists of two coils 
placed in series, one of which is movable, AB^ and the 
other fixed, CD. When a current 
passes through these coils they be- 
come magnetized, repulsion of the 
movable coil occurring and the mova- 
ble coil turning on a central axis. The 
motion of the movable coil is resisted 
by a spiral spring, and it is Hmited in 
its motion by two stops. By turning 
the spiral spring or rewinding it by a 
knob on the top of the instrument, the 
movable coil may be brought back to 
zero while the current is passing 
through it and when a deflection has 
occurred. The amount of twist nec- 
essary to bring the movable coil back 
to zero is indicated by the motion of a 
pointer over a dial. The magnitude of 
the current that passes varies as the square root of this 
deflection. The value of the current is expressed in terms 
of a constant K^ and the deflection d. 

If the direction of the current changes, the polarity of both 
coils will change, and the deflection will still remain the 
same. For this reason, as previously stated, the instrument 
was used for many years to measure alternating currents. 
At present, however, it is not being used to any great 
extent, as the modern portable instrument indicates the 
current directly without making any adjustments and 
without making any calculations or using any constants. 




Fig. 44. — Electro- 
dynamometer. 



ELECTRO-MA G METIS M 3 1 

TJie Effect of an Iron Core in a Helix. 

Experiment 16. Pass a current of electricity of about 5 amperes 
through a helix, as in Fig. 40, so that it will just deflect a small compass 
needle placed at some distance from it, say a foot. Introduce a soft 
iron core into the hehx, and notice that the compass is attracted to 
a much greater degree. When the iron core is added, it becomes mag- 
netized, its molecules arranging themselves so as to help the magnetic 
strength of the circuit, the combined effect of core and coil being several 
thousand times greater than if the coil alone were present. The core, 
in addition to its magnetizing effect, facilitates the passage of these lines 
of force. In large generators, motors, etc., and in both direct and alter- 
nating current apparatus, this characteristic of soft iron is utiHzed to 
excellent advantage. In fact, upon this characteristic the growth of the 
electrical industry has depended more than upon anything else. If it 
were necessary to take from our main generators the iron used in their 
construction, their efficiency would be so much reduced that the whole 
electrical industry would be crippled. 

Permeability, Saturation, of Iron. — Experiment shows 
that the presence of an iron core in a helix increases the 
apparent magnetic strength of the circuit. Whether it is 
due to the fact that the molecular structure of the iron 
is rearranged, thus adding its molecular magnetism to the 
magnetism of the circuit, or whether it is due to the fact 
that the iron serves as a better conveyor of lines of force, 
there can be no doubt that the magnetization is greatly 
increased by the presence of the iron core. This charac- 
teristic of iron is termed permeability. It is not the same 
in magnitude for all kinds of iron. Some iron increases 
the magnetization much more than others. The perme- 
ability is not directly proportional to the magnetizing force 
used or to the current passing through a given winding, 
but varies with the previous magnetic condition of the 
circuit. At first, the increase in lines of force with a given 
magnetization is very great. Later, the curve turns at the 



32 



EXPERIMENTAL ELECTRICITY 



knee of the magnetization curve ; and beyond this point, 
although the increase in magnetization is quite great, the 
increase in the number of lines of force in the circuit is 
quite small. In other words, the iron has become saturated. 
According to the molecular theory of magnetism, the mole- 
cules of the iron when saturated have been rotated about 
as far as they will go, and it requires in this condition an 
infinite magnetizing current to bring about the final slight 
increase in magnetization possible. 




MAGNETIZING FORCE 

Fig. 45. — Magne 
tization Curve. 



Experiment 17. Obtain a magnetization curve, 
using same apparatus and same set-up as in Experi- 
ment 6, except start with zero magnetization and 
turn on one i6-candle-power lamp, etc., up to the 
last 50-candle-power. For each increase of current, 

throw reversing switch and obtain total change of flux. Plot curve as 

Fig. 45. (See footnote. Experiment 6.) 

The Electro-magnet. — An electro-magnet is a helix with 
an iron core. Electro-magnets may have various shapes 
and forms, and may consist of one or more solenoids. A 
large coil of 3000 turns (Long Tom) when provided with 
an iron core will enable one to perform many interesting 
experiments. 



nil 



Experiment 18. Insert the soft iron core in the electro-magnet 
place a glass plate over the end of the bar, and 
support the magnet and plate in a vertical posi- 
tion, (i) Sprinkle about a pound of iron filings 
over the glass. The filings may be drawn out into 
fine threads, Fig. 46, forming a hairlike growth. 
When the circuit is opened the filings will fall, and 
when it is closed they will stand up again. This 
alternation may be repeated several times. (2) On 
placing the hand under the filings and exciting the 
circuit, the observer will note the strong pull of the filings. (3) Nails 
thrown at the magnet so that they will hit it or come near to it will be 



Fig. 46. — Filings 
showing Strength 
of Electro-magnet. 



ELECTRO-MA GNETISM 



11 



attracted and stand out in the direction of the magnetic field, Fig. 47. 

(4) A small piece of diaphragm, such as may be taken from a tele- 
phone receiver or cut from a ferrotype plate, 

if placed upon the end of the pole, will rise on 

edge, as in Fig. 48, so as to accommodate the 

greatest possible number of lines of force 

through the circuit. The piece of diaphragm 
is of more service to the 
magnetic circuit when up- 
right than when flat upon 
the pole because it tends to 
decrease the reluctance of 
the circuit. (5) The mag- 
netic circuit may be traced by means of a test nail 

fastened to a fine thread which is held near the magnet and passed from 



d. 



^z 






2:. 



^ "^ 



Fig. 48. — Dia- 
phragm and 
Magnet. 



Fig. 47. — Nails showing 
Distribution of Flux of 
Magnet. 



one end of it to the other. Fie 



^ ^ od. 



/•• 



f-^ 



49. (6) The principle of the Edison 
ore separator may be readily shown. 
Fig. 50, by mixing some filings with 
sand and allowing them to fall in a 
stream near the pole of the magnet ; the 



fihngs will cHng to 
the magnet to 
which they are at- 



1:;:., 



SAND 
AND 
FILINGS 



=r: FILINGS 



Fig. 49. — Tracing the Field. 

tracted, whereas the sand will fall, forming a pile. 
Edison's apparatus was so arranged that the iron 
ore mixed with sand was allowed to fall down a 
chute. Powerful magnets diverted the iron parti- 
cles to one side, whereas the sand passed on. In 
this way a large portion of the sand was removed, so that the remain- 
ing mixture was workable. 



Fig. 50. — Principle of 
Edison Ore Sepa- 
rator. 



The Blow-out Magnet. — A very important application 
of electro-magnetism, making use of the principle that a 
conductor carrying a current is attracted by any electro- 
magnet, is the blozv-oiU magnet. When an arc is formed 
and maintained, the current of electricity traverses the arc 
through the conducting vapor, which may be metallic 
vapor, carbon vapor, etc. An arc, therefore, is a conduc- 



34 



EXPERIMENTAL ELECTRICITY 



tor carrying a current and also a flexible conductor, and if 
the arc is placed in the vicinity of the electro-magnet, it 
will be sucked out or blown out, depending upon the direc- 
tion of the current, being quickly extinguished with a 
sharp snap. This principle is used in railway trolley con- 
trollers, where a hinged pole of a magnet is placed over 
the contact fingers to extinguish the arcs formed on open- 
ing and closing the contacts. In the case of the new types 
of controllers for trolley cars, and in the multiple unit con- 
trol for railway operation, the main circuits are closed by 
contactors placed under the car. 



Experiment 19. Place a pole of a compara- 
tively strong bar magnet near an arc formed 
between two carbons, Fig. 51, and notice how 




Fig. 51. — Arc 
guished with Magnet 



extm- quickly the arc is extinguished. 



116 VOLTS 

4- 



=ill 



a 



Fig. 52. — Arc extiu' 
guished with Electro 
magnet. 



Experiment 20. Arrange the circuit of a large 
magnet so that it can be opened and closed near 
the pole of the magnet, as in Fig. 52. Open the 
circuit and notice the rapidity 
with which the arc is extin- 
guished. 



Applications of Elec- 
tro-magnetism. — Among practical appli- 
cations of the electro-magnet may be men- 
tioned the field coils of motors, dynamos, 
parts of arc lamp mechanisms, electric 
bells, and relays and sounders for telegraph 
work. The electric bell. Figs. 53, 54, 55, 
consists of a small pair of magnets con- 
nected to two binding posts through a con- 
tact A, Fig. 55. When the circuit is closed by a spring 
pressing the armature against the contact A, the magnet 
draws the armature over, the clapper striking the bell. 




Fig. 53. —Electric 
Bell (M. E. S. 
Co.). 



ELECTRO-MAGNETISM 



35 



This opens the circuit at A, and the spring returns the 
clapper armature to contact position again. This produces 
a continuous ring. A buzzer operates — 

upon the same principle as the electric 
bell, except that the bell and clapper are 
removed, the sound being made by the 
movement of the armature. The tele- 
graph system consists of a line, Fig. 56, 
a series of transmission keys, Fig. 57, and 
a series of magnets termed relays, Fig. 
J—. 58, all connected in cir- 

"^^^^ni — ^ ^^^^ ^\!Cii a battery, as in 

^^^J^^ © Fig. 56. The line is 
grounded at both ends 
Fig. 55. — Circuit of of a whole Series of sta- 

Eiectnc Bell. tious. Gravity batteries are used, as the 
system is a closed circuit system. Operating any one of 
the keys will cause all of the relays on the system to draw 




Fig. 54. — House Bell 
Circuit (M. E. S. 
Co.). 



j/ — I 
lis 






tip 



Fig. 56. — Telegraph Line. 

their armatures to the poles of the magnets. In so doing, 
the armature closes a local circuit consisting of a battery 
and a sounder, Fig. 59, which intensifies the click of the 
relay that might other- 
wise be indistinguish- 
able. The sounder is 
simply another pair of 
magnets with a heavy 
armature. As the cur- 
rent which passes Fig. 57. — Transmission Key (M.E.S. Co.). 




36 



EXPERIMENTAL ELECTRICITY 




Fig. 58. — Relays (M.E. S.Co.). 



through all of the relays is very small, they are wound with 

many turns of fine wire 
to secure sufficient mag- 
netic effect. The resis- 
tance of the relays is 
therefore great, being 
from 80 to 300 ohms. 
The resistance of the 
sounder is much less 
and may be anything from 5 to 20 ohms. To send a mes- 
sage the operator waits until the 
line is clear, and then he opens 
the short-circuiting switch on his 
key, and this opens the main line. 
He presses the key, making con- 
tact, and a spring returns the key 
when the pressure is removed. 
The length of time of contact may be varied, producing 
dots and dashes. 




Fig. 59. — Sounder. 




Fig. 60. — Electric Motor. 



ELECTR 0-MA GNETISM 



37 



The Electric Motor. — The electric motor is discussed 
at great length in a subsequent chapter, so it is advis- 
able at this point to give only a general idea of its 
electro-magnetic performance. An electric motor, Fig. 60, 
has two elements, one 
of which is movable and 
the other fixed. The 
movable element trans- 
mits the power and is 
termed the armature, Fig. 
61 ; the fixed element is 
termed the field. The 
field circuit surrounds the armature circuit, and consists of 
a frame supporting an even number of poles or electro-mag- 




FlG. 61. — Armature of Motor 
(Westinghouse). 




Fig. 62. — Field Coils of Alotor (Westinghouse). 

nets, Fig. 62. All electric motors possess two elements, 
whether they are of the alternating current type or the 
direct current type. A direct current is one which con- 



38 



EXPERIMENTAL ELECTRICITY 




t'lG. 63. — Inducuoa Aiuior 
(G. E. Co.). 



tinues to flow in the same direc- 
tion once it has established itself, 
whereas an alternating current 
is one which is continually re- 
versing its direction of flow with 
a regular periodicity. With an 
alternating current motor, it 
may be that the rotating ele- 
ment, or rotor, A, Fig. 63, con- 
sists simply of a number of cop- 
per bars assembled around the 
periphery of a circular core in 
slots, and short-circuited at its 
extremities. An alternating cur- 
rent motor employing such a ro- 
tating element is termed an 
induction motor (see chapter on 
induction motor). There are 
two types of direct current motor 
in common use, the shunt and 
the series motor (see subse- 
quent chapters). Modifications 
of these two types are the com- 
pound motor and the interpole 
motor. The armature of a direct 
current motor consists of an 
iron core made up of laminated 
sheets of iron mounted upon a 
spider perpendicular to the shaft. 
In the perimeter of this core are 
slots in which are placed the ar- 
mature conductors. The termi- 
nals of the armature coils are 



ELECTRO-MA GNETISM 



39 




connected to copper segments insulated from each other 
with mica, constituting the commutator, Fig. 64. The 
coils are wound upon the arm- 
ature so as to form the proper 
magnetic circuits when a cur- 
rent is sent into the commu- 
tator by means of the brushes. 
There are many forms of arma- 
ture windings which have been 
developed by various design- 
ers, and to these the reader is fig. 64.— Commutator (Westing- 

ref erred. The simplest of all ^°^''^- 

is the common series winding illustrated in Fig. 65. 
This consists of one or more sets of coils connected 
in parallel between adjacent commu- 
tator segments to form a closed series 
winding. When the current enters the 
armature and field circuits, they be- 
come magnetized, forming poles and 
causing rotation. See Fig. 66 for 
magnetic circuits. The brushes are 
made of carbon or copper, carbon be- 
ing preferable. The substitution of the carbon brush 
for the copper brush was a great step for- 
ward in electric railway operation and did 
much towards establishing electric rail- 
roading on a substantial basis. As the 
armature rotates, the commutator moves 
under the brushes, and this maintains the 
same relative magnetic relations of arma- 
ture and field. The direction of the current 
in the coil undergoing commutation changes at the same 
time. 




n^coiL 
rri coppE 

I I lSEG^^E^ 



Fig. 65.— Series Wind 
ins. 




Fig. 66. — Rota- 
tion of Armature. 



40 



EXPERIMENTAL ELECTRICITY 



Fig. 67. — Shunt. 



In the shunt motor the armature and field coils are in 
multiple with each other, Fig 6'j, whereas in the series 

motor the coils are in 
+ - 4. - series with each other, 

Fig. 6Z. The armature 
of a direct current mo- 
tor is usually repre- 
sented by a small circle 
Fig. 68. — Series. with two brushes press- 
ing upon it, — o — , the 

field circuit being represented by the wave ^ , Fig. 69. 

A shunt motor is used for constant speed work, whereas a 
series motor is used for variable speed ^-. 

operation. The shunt motor need not ^^^ ^ —Armature 
necessarily be rigidly connected to its and Field Sym- 
load, whereas this is a necessity with the 
series motor. The series motor exerts its greatest tractive 
force when starting, and is therefore particularly service- 
able for elevator 
and railway oper- 
ation. Should 
the load of a 
series motor be- 
come detached 
from it, as by a 
belt slipping off 
the motor pul- 
ley, there is dan- 

FiG. 70. — Cross Section of Armature (Westinghouse). ^^ ^^ motor 

armature's damaging itself by the speed of rotation becom- 
ing excessive. 

A cross section of an armature of a Westinghouse shunt 
motor is shown in Fig. 70. 




ELECTRO-MAGNETISM 



41 



E 



m 




LS 



S" 



Fig. 71. — Thomson Inclined Coil Ammeters. 



Thomson Inclined Coil Ammeters and Voltmeters. — The 

Thomson incHned coil voltmeters and wattmeters operate on 
the dynamometer principle, whereas the Thomson amme- 
ters operate on the magnetic vane principle. This consists 
of a single solenoid 
mounted at a slight 
angle. With the am- 
meters, Figs. 71, 72, 
a small iron vane is 
mounted at the same 
angle as the solenoid 
upon a vertical bronze 
shaft, which terminates in polished sapphire jewel bearings. 
When turned, the vane enters more and more into the sole- 
noid, until its axis be- 
comes parallel to the 
central axis of the coil. 
A pointer attached to 
the axle moves over a 
graduated scale as the 
vane turns. The vane 
turns when the coil is 
energized, as the iron 
vane tends to enter more 
and more into the mag- 
netic field of the coil to 
help accommodate the 
lines of force passing through it. A flat spiral spring of 
highly tempered phosphor-bronze, mounted upon the axle, 
returns the needle to zero position after a deflection. The 
swinging of the needle is reduced to a minimum by an 
air vane damper mounted under the pointer and fastened 
to the axle. Excessive oscillation of the needle may be 




Fig. 72. — Thomson Inclined Coil Ammeter. 



42 



EXPERIMENTAL ELECTRICITY 



resisted by a friction damping device operated by a small 
push button. The whole instrument, Fig. 72, is quite light 
and is inclosed in a gun-metal case ; owing to its lack of 
permanent magnets it retains its calibration over a consid- 
erable period and is not so likely to be affected by stray 
fields. The instrument is not affected to any extent by 
variation of frequency, wave form, or power factor. It 
lacks, however, the dead-beat characteristic of a perma- 
nent magnet type of instrument. 

The Thomson inclined coil voltmeter is similar in con- 
struction to the ammeter, except that an inclined coil is 
substituted for the metal vane, the meter acting on the 
dynamometer principle. 

Thomson Inclined Coil Wattmeter. — This instrument 
has two coils — a fixed low resistance coil, and a movable 
high resistance potential coil which is mounted inside of 
the current coil. Both coils are inclined as in the incHned 

coil ammeter. When en- 
ergized, the movable po- 
tential coil tends to set 
itself parallel to the cur- 
rent coil. The other 
structural features are sim- 
ilar to those of the ammeter 
and voltmeter. 

Weston Indicating Watt- 
meter. ^ In this type of 
instrument, Fig. 73, there 
are two coils, one fixed, the 

Fig. 73. -Weston Wattmeter Movement. ^^j^^^. movable; the mov- 
able coil turns inside of the fixed coil at right angles to it. 
The fixed coil or current coil has a low resistance and is 
connected to two binding posts which are placed in series 




ELECTRO-MA GNETISM 



43 




Fig. 74. —Tractive Magnet built by Cutler Hammer Clutch Co 



44 EXPERIMENTAL ELECTRICITY 

with the drcuit when used. The movable coil, or potential 
coil, has a high resistance and is connected through two spi- 
ral springs to two other binding posts, which are placed in 
multiple with the consuming device whose energy con- 
sumption is being measured. The instrument is very ac- 
curate and operates on the dynamometer principle, but does 
not possess the dead-beat characteristic of other types of 
Weston instruments. 

Traction Electro-magnets. — An extensive use of electro- 
magnets for tractive purposes is being developed at the 
present day by some companies, notably the Cutler Ham- 
mer Co. One of these magnets is shown in Fig. 74, where 
pig iron is being unloaded from a car. 



QUESTIONS 

1. How does an electro-magnet differ from a permanent magnet; 
and in what respect are they similar? 

2. Of what advantage is the iron core of an electro-magnet, and to 
what extent does it affect the apparent strength of the magnet ? Can 
you explain this phenomenon? 

3. Why is an electro-magnet consisting of two solenoids preferable 
to one solenoid? 

4. What do we mean by the terms magnetizing force and flux 
density ? 

5. Give a sketch showing how you would determine the polarity of 
a solenoid if you knew the direction of the winding and the direction of 
the current through it? 

6. How would you proceed to magnetize a piece of steel with a 
solenoid ? 

7. Describe the principle of operation of the Weston alternating 
current ammeter, wattmeter, and the Thomson inclined coil ammeter of 
the General Electric Co. 

8. Which will make the stronger solenoid. 100 turns of wire with 
I ampere passing through it, or 10 turns of wire with 10 amperes passing 
through it? Is there any difference approximately? 



ELECTRO-MA GNETISM 



45 



9. Why is the iron plunger of a solenoid drawn up into the solenoid 
when the coil is energized? 

10. Give a few practical illustrations of electro-magnets, and explain 
their function. 

11. If the winding of a solenoid were partially short-circuited and 
the current passing through it was the same as before the short circuit 
occurred, how would it affect the strength of the solenoid? 



CHAPTER III 

ELECTRO-MAGNETIC INDUCTION — THEORY OF THE 
DYNAMO 

In 1 83 1 Michael Faraday discovered the principle of 
electro-magnetic induction. He noticed that if a loop of 

-, ^ — wire is moved in a mae^netic field 

N f-U%— -^J^-"^^ so as to cut the lines of force of 
fk;. 75. — Method of generat- that field, a Current of electricity 
ingZeroE.M.F. circulates in the coil, due to an 

electro-motive force induced in it. If the wire is moved 
in a direction parallel to the lines of force as in Fig. 75, no 
e. m. f . is induced in the loop, as the 
wire must cul the lines of force. A ( ) I J/J { \ 

loop of wire thrown loosely over a V.^Z V„y^ 

cyHnder, as in Fig. j6y would repre- fig 76. - Lines of Force not 

T - . . . Interlinked. 

sent a loop of wire m a magnetic 

field, but the lines of force would not be 
In interli7tked with the loop. 



T 

I i I Experiment 2 1 . Take a large coil having many turns 

(Long Tom) and connect this coil to a galvanometer. 
Quickly plunge a bar magnet into the coil, Fig. jj, and 
notice that the galvanometer needle deflects. Notice 
also that the deflection occurs only when the magnet is 
Fig. 77.— Gen- j^ motion. Move the magnet in very slowly, and no- 
F^m"^f ^^ ^^^^ ^^^^ practically no deflection occurs. Move the 
magnet in very quickly, and notice that the deflection 
becomes very great. Notice also that when the magnet is extracted from 
the coil, the deflection of the galvanometer is in the opposite direction 
from that when the magnet is moving into the coil. Move the magnet 

46 







fn\ l(^ 


.oTS: [^ "^ 


r^-^ 


M i 

+ 116 VOLTS — 

i.e. 

Fig. 78. — Generating an 
E. M. F. 


re 


similar. Notice the 



ELECTRO-MAGNETIC INDUCTION 47 

into and out of tlie coil, causing the galvanometer needle to deflect, 
first in one direction, and then in the other, thus generating an alter- 
nating e.m.f. 

Experiment 22. Connect a coil of many turns (Long Tom) to a 
constant source of potential, the ii6-volt Edison service, Fig. 78. Con- 
nect a long wire which can be wound up in a 
coil of 20 loops to a galvanometer. With but 
a single loop, equivalent to a straight conduc- 
tor, move the wire across the top of the coil so 
as to cut the lines of force emanating from the 
pole, and notice deflection of galvanometer. 

Experiment 23. Repeat all of the previous 
experiments with the coil as with the magnet, 
the excited coil taking the place of the per- 
manent magnet, and notice that the results are similar. 
efl"ect of speed and the direction of the moving coil. ' 

Experiment 24. Increase the number of turns in the loop to 2, 4, 
8, 10, etc., moving coils of the loop across the excited large coil, main- 
taining the speed of motion about the same in each case, and notice 
that the deflection of the galvanometer needle is proportional to the 
number of turns. 

Experiment 25. Insert a resistance in series with a coil of such mag- 
nitude that the current passing through the coil will be cut down to one 
half, and compare the magnitude of deflection for the same speed of 
motion of the coil for 5 turns in this case, with the same number of turns 
in the previous case. The deflection in this case will be approximately 
one half of that in the former, as the magnetic strength of the solenoid 
has been reduced to one half. 

Generation of an Electro-motive Force. — From the previous 
experiments it will be noted that the magnitude of the elec- 
tro-motive force generated by moving a coil in a magnetic 
field varies with the speed of motion, with the number of 
turns in the coil, and with the strength of the magnetic 
field. If a magnetic field of a certain number of lines of 
force of say 100,000,000 be cut by a single loop of wire in 
I second, i volt ivill be generated m the wire. If this field 
be cut in \ second by the single loop, 2 volts will be gener- 



48 EXPERIMENTAL ELECTRICITY 

ated. If it be cut by 2 turns in 2 seconds, i volt will be 
generated. If the field strength be reduced to 50,000,000 
lines of force and they be cut by i turn in i second, \ volt 
will be generated. The electro-motive force or voltage gen- 
erated varies directly as the total number of lines of force 
cut in one second. When 100,000,000, usually designated 
as 10^ called 10 to the 8th power, lines of force are cut in 
I second, i volt is generated. This relation is conveniently 
expressed in the simple form : 

^ = 8 

10^ 

Where £" = e. m. f. in volts, 

5 = revolutions of coil per second, 
N = number of turns in loop, 
^ = total number of lines of force. 

This is for a two-pole machine. For any other number of 

pairs of poles the 
e. m. f . will be di- 
rectly proportional 
to the number of 
pairs of poles. A 
dynamo electric 
machine used to 
generate an elec- 
tric current is 
termed a ge^terator. 
Generators. — A 
direct current gen- 
erator is almost 
FiG.79.-RotaryConverter(G.E.Co.). identical in Con- 

struction with a direct current motor. If the armature 
of a shunt motor be driven by an external force and the 




ELECTRO-MAGNETIC INDUCTION 49 

field winding be separately excited, an electro-motive 
force will be generated, and the motor will be operating as a 
generator. A number of generators have been built in which 
about two volts per foot of active conductor are generated. 
It is well to remember that the electro-motive force which 
is generated in the winding of any electrical generator is 
alternathig in cJiaracter and that this electro-motive force in 




Fig. 80. — Motor Generator (Westinghouse). 

order to be direct must be rectified by a commutator. If 
two slip rings are connected to the windings at two points 
directly opposite to each other on a two-pole machine, an 
alternating cicrrent geiierator will result. A machine may 
be constructed with a commutator on one end of the arma- 
ture and slip rings on the other end. Such a machine, when 
driven and when its field coil is excited, will yield direct cur- 
rent at the commutator and alternating current at the slip 
rings, being what is known as a double ciLrrent generator. 
A double current generator may be operated as a motor 
on its direct current side and would supply alternating 
current at the alternating current slip rings. It would be 
termed an inverted rotary converter. A rotary converter 



so 



EXPERIMENTAL ELECTRICITY 



is usually operated from an alternating current source 
of supply on the alternating current side and delivers 




—116 VOLTS + 

d.c 



? 



Fig. 8i.— Motor Generator (G. E. Co.). 

direct current at the commutator side, being thus a normal 
rotary co7tvei'ter^ Fig. 79, or synchro7ioiis converter. If the 
double current generator be supplied with two separate 
windings with the same field, each wind- 
ing having a separate commutator con- 
nected to different number of armature 
turns, such as five times as many turns 
on one armature winding as the other, 
the machine can be operated as a motor 
from one winding, yielding a pressure 
five times as great from the other arma- 
ture winding. Such a machine is called 
a dynamotor. Where the windings each 
have separate field coils, the machine is 
called a motor generator^ Figs. 80, 81, 
82. If two generators be operated in a 
series parallel combination, on a three- 



-vwvn 



^AA/W-' 



GENERATOR 



<C^ 



SHAFT 

-\/\f\J\f\fsf-\ts/\[\r 



Fig. 82. — Circuit 

Motor Generator 



of 



ELECTR 0-MA GNETIC IND UCTIOM 



51 



wire system as in Fig. 83, they tend to balance the system 
and are termed balancers. The machine on the heavier- 
loaded side acts as a generator, while the 
machine on the Hght-loaded side acts as 
a motor. 

Variation of E. M. F. of Generator. — The 
e. m. f. of a generator may be varied by 
increasing its speed of rotation, or by in- 
creasing the number of armature conduc- fig 
tors in series, or by increasing the field 
excitation. Increasing the field excitation mai 
by decreasing resistance in series with it, 
as in Fig. 84, is the customary method 
used in practice. 




+ ± - 

83. — Balancers. 



^Qj 




EXCITING CIRCUIT 

Fig. 84.— Field Cir- 
cuit of Generator. 



Experiment 26. Take a small motor generator 
and separately excite the generator field coils 
through a field rheostat, Fig. 82. Have a rheostat 
also connected in series with the field coils of the 
driving motor so that the speed of the motor may 

be varied. Connect a projecting voltmeter or an ordinary voltmeter 
across the armature terminals so that the voltage generated will be indi- 
cated. Vary speed of motor, noting speed with speed indicator. Note 
that the voltage generated varies directly as the speed. Vary the excita- 
tion of the generator by varying the field rheostat of the generator, and 
note that the voltage generated varies with the excitation, but not quite 
directly. Motor is not connected to mains in cut. 



Magnetization Curve of a Shunt Dynamo. 

Experiment 27. With a set-up similar to that in the last experiment, 
keep the speed of the generator constant. Vary the excitation of the 
generator from zero to maximum and then back to zero, reading the 
voltage generated and also the current passing through the field circuit. 
An ammeter should be placed in series with the field circuit. In taking 
observations be careful to make the steps of the rheostat adjustment 
continuous. If one finds that one has moved the rheostat handle too 



52 



EXPERIMENTAL ELECTRICITY 



VOLTAGE 
GENERATED 



far, do not move it back and then forward, since by so doing an imper- 
fect curve will result, due to residual magnetism. From this experiment 

a curve will be obtained, Fig. 85. It 
will be noted that the descending 
curve lies above the ascending curve. 
This is due to residual magjietism. 

Residual Magnetism. — Re- 
sidual magnetism is a natural 
magnetic condition of iron 
which appears to be present in 
most manufactured sheets used 
in building up the lammated 
field and armature circuits. Small soft iron wires may be 
made highly magnetic by twisting them in a lathe until 
they become hardened and break. 




MAGNETIZING CURRENT 



Fig. 85. — Magnetization Curve. 







o 



Excitation of Generator, 



Experiment 28. Connect a voltmeter to the armature terminals of a 
small generator. Fig. 86.- Disconnect the field terminals and do not 
excite field circuit. Drive the arma- 
ture of the generator and notice that, 
although there is no exciting current 
present, a voltage of about 5 volts 
will be generated. This e. m. f. is 
generated owing to the residual mag- 
netism of the iron. 

Experim,ent 29. With the same set-up as before connect the armature 
and field circuits together to form a shunt connection, Fig. 86, and 
start the generator operating. Field terminals should be so connected 
that the machine will not build up. If it does build up, reverse the field 
terminals as in Fig. 86. When this connection is made it will be noted 
that the previous readingof 5 volts will be reduced to 3 volts orthereabouts- 
The 5 volts previously generated tend to send a very small current 
through the field circuit in the wrong direction. This small current 
tends to demagnetize, or "knock out.'" the residual magnetism. If it 
were possible to reduce entirely this 5 volts to zero, it would obviously 
leave no residual magnetism to build up on the other side of zero. 



ELECTRO-MAGNETIC INDUCTION' 53 

Experiment 30. Make the proper field connection to the armature, 
and note that voltage builds up. 

Experiment 31. With the dynamo field circuit connected so that it 
will not build up, change the direction of rotation, and note that the 
machine does build up, — why ? 

The relation of direction of rotation to the building up of 
the field circuit is made use of to excellent advantage 
in some railway electric car lighting systems as used by 
steam roads. Instead of changing the field terminals 
when the direction of motion of the car changes, the brush 
holders are so mounted that they will be carried forward or 
backward from one neutral plane to the other by the friction 
of the brushes, their motion being limited by two stops. 
Irrespective of the direction of motion of the car, the arma- 
ture and field terminals of the generators always have the 
proper connection for building up. This system is used by 
the Bliss Car Lighting Company. 

Mutual Induction. — When an electro-motive force is gen- 
erated in a coil by the presence of a magnetic field produced 
by another coil, the effect is termed mutual 
induction. In order that the effect of one 
coil may be felt upon the other coil, the 
lines of force must be interlinked with it. 



r<§> 



^ 



Experiment 32. Place a magnet in a large coil 
(Long Tom), and connect the coil to a galvanom- 
eter. Allow a small piece of diaphragm. Fig. 87, 
to come into contact with the magnet and then 
suddenly draw the diaphragm away from the mag- Fig, 87. — Principle 
net. A deflection of the galvanometer will occur, °^ Induction. 

due to a change in the magnetic condition of the circuit. Mutual induc- 
tion is present in transformers. 

Foucault Currents. — A copper disc. Fig. '^^, rotated be- 
tween the poles of a permanent magnet has an electro-motive 




54 EXPERIMENTAL ELECTRICITY 

force induced in it and is equivalent to a straight wire 
carrying a current. This electro-motive force causes a cur- 
rent to circulate in the disc, and the current produces a 
magnetic field which tends to resist rotation. Such cur- 
rents are called Foiccault or eddy currents. The reason 
that it is necessary to laminate armatures 
of dynamos and motors is to reduce to a 
minimum the circulation of these eddy 
currents. The currents circulate parallel 
to the shaft of the armature, and tend to 
^ „„ ,_ heat the armature circuit. A small coat- 

FlG. 88. — Wattme- 
ter Disc with ing of shellac on the ordinary laminations. 
Magnets. ^j. ^j-^^ hard surface coating which is being 

formed on modern laminated iron is sufficient to insulate 
these currents and reduce the loss to a very small amount. 

Experiment 33. Connect a pair of small electro-magnets to a few 
batteries so that the magnets will be strongly magnetized. Suspend a 
small piece of sheet copper, such as may be cut from the copper elec- 
trodes of a gravity cell, so that the copper disc can swing back and 
forth between the poles of the magnet. With the magnets excited, 
notice that if the copper strip is drawn to one side and released. 
Fig. 89, it will be suddenly arrested in its motion as it tends to pass the 
poles of the magnet. 

Experiment 34. Cut another strip from the same sheet of copper 
of the same dimensions and slit the strip with staggered cuts, as in 
Fig. 90, so as to insulate the path of the eddy currents. Suspend this 
piece in place of the strip used in the previous experiment, excite the 
magnets, draw piece to one side, release it, and notice that it does not 
stop in its motion past the magnets. (Both of these experiments are 
readily adaptable to a horizontal lantern.) 

Application of Foucault Currents. — One of the greatest 
practical applications of foucault currents is in the Thom- 
son Recording Wattmeter, Fig. 28. In this instrument the 
magnets are mounted in the base of the wattmeter, so 



ELECTRO-MAGNETIC INDUCTION 



55 





- Principle of Eddy- 
Currents. 



that a disc can move between them. This disc, either of 
copper or aluminium, has eddy currents generated in it 
which act as a drag or load upon 
the meter, the load being directly 
proportional to the speed of rota- 
tion of the meter. In calibrating 
the wattmeter it may be that the 
meter is either fast or slow. By 
moving the magnets in or out over 
the disc, a variation of 15% in the 
meter speed may 
be made. In in- 
stalling such me- 
ters, care must be 
taken to see that 
they are not af- 
fected by stray fields of other circuits near 
by. A heavy short circuit in the meter 
also tends to affect the magnets, some- 
times strengthening one pole of a magnet and decreasing 
the strength of the other pole. Although the motor type 
of meter presents a few of these difficulties, it is found in 
practice that when properly maintained and inspected such 
a meter is very accurate and gives practically no trouble. 
See chapter on recording wattmeters for further description. 
Practical Applications of Induction. — Many practical 
applications of induction exist in transmission systems. 
We have the induction coil. Figs. 91, 92, consisting of a 
primary and a secondary winding, the primary circuit hav- 
ing an interrupted current sent through it. The induction 
coil is used extensively in telephone circuits, Fig. 93, and 
in wireless telegraph circuits. Fig. 94. The principle of the 
wireless circuit is indicated in Fig. 94, where a coherer is 



Fig. 90. — Insulation 
of Eddy Currents. 



56 



EXPERIMENTAL ELECTRICITY 



TRANSMITTER 




Fig. 92. — Shocking Coil, 



TRANSMITTER 



TRANSMITTING STATION 




!/ 



RECEIVING STATION 



LOCAL BATTERY 



r 



PLUGS „ 
P,L,NOS M 



Q94M] : 

LOCAlI BATTERY 



£ 



i 

Fig. 94. — Wireless Circuit. 



ELECTRO-MAGNETIC INDUCTION 



57 




Fig. 95. — Wireless Sending Apparatus. 




Fig. 96. — Wireless Receiving Apparatus. 




58 EXPERIMENTAL ELECTRICITY 

shown which consists of two small pellets inclosed in a 
tube containing nickel fiHngs. Ordinarily the circuit shunt- 
ing the coherer is interrupted, but when 
a wave is sent out from the sending sta- 
tions, the particles of nickel stick to- 
gether, closing the shunt circuit. When 
this happens, an armature is drawn up, 
striking a bell, and also the coherer 
tube, giving a signal and also interrupt- 
ing the circuit. The large commercial 
sets, such as used by the Marconi Com- 
FiG. 97. — Desk Tele- pauy, are more complicated than the 
phone. simple apparatus shown in Figs. 95, 96. 

This apparatus, however, is good for experimental purposes 
and may be purchased from O. T. Louis Co., 59 Fifth Ave., 
New York City. A convenient form of telephone, termed 
a desk telephone, is shown in Fig. 97. The various circuits 
shown are self-explanatory. 

QUESTIONS 

1. Who discovered the principle of electro-magnetic induction ? 

2. What does the term eddy ai7'rents mean? 

3. Explain the action of eddy currents in a wattmeter disc and show 
how temperature changes in the disc affect the meter's accuracy. 

4. What is the object of laminating dynamo electro machines? 

5. Cite some examples of where the magnetic field due to one 
wire has affected the operation of a circuit in the vicinity. 

6. Explain the theory of the generation of electro-motive forces. 

7. Why are pole dampers placed on the pole faces of rotary con- 
verters and alternating current generating apparatus? 

8. How does the principle of induction enter into the operation of 
a transformer? 

9. How can a resistance be wound so that it will be non-inductive ? 
10. Show how induction may be likened to inertia. 



CHAPTER IV 

OHM'S LAW 

In dealing with a direct current circuit the terms electro- 
motive force ^ current, and resistance are frequently met with. 
Each of these terms has a unit, a symbol, and a measuring 
instrument associated with it. They may all be tabulated 
as follows : 



Term 


Unit 


Symbol 


Measuring 
Instrument 


Electro-motive Force 

Current 

Resistance. . . . 


Volt 

Ampere 

Ohm 


E 

I 
R 


Voltmeter 

Ammeter 

Ohmmeter* 



Resistance. — Resistance may be discussed first, as it is 
most readily understood. Resistance may be defined as an 
opposing force which has to be overcome in order to cause 
a flow of electrical energy. In so doing a certain amount 
of energy is continuously transformed into heat by the 
resistance so long as the flow of electrical energy continues. 
The amount of energy dissipated by the resistance in the 
form of heat is proportional to the time the energy is flow- 
ing, and to the magnitude of the current which is passing 
in the circuit. Resistance in an electrical circuit is some- 
what analogous to the resistance of water pipes to the pas- 

* In measuring resistance there are many other methods used besides the ohm- 
meter, as may be noted in the chapter on Electrical Measurements, the ohmmeter 
being used infrequently. 

59 



6o 



EXPERIMENTAL ELECTRICITY 



sage of water. The greater the length of the conductor, the 
greater is the resistance, and the greater the cross section 
of the conductor, the less is the resistance. The energy 
dissipated by the resistance is usually in the form of heat, 
although it may be accompanied by a transformation into 
mechanical energy or into radiant energy in the form of 
light. The symbol "i?" is usually used to designate resist- 
ance and also the wave ^....^^. An adjustable resistance has 
a movable contact placed on the wave ___. The unit of 




Fig. 



Car Resistance. 



resistance is the oJivi. Numerically it is equal to the resist- 
ance of a uniform column of mercury 106.3 centimeters long, 
weighing 14.4521 grams in mass, at 0° centigrade. Some 
idea of the dimension of the resistance may be had from the 
following : 

1. A 1 6-can die-power ii6-voIt carbon lamp has a resistance cold of 
approximately 500 ohms, and when hot a resistance of 250 ohms. 

2. The resistance of 1000 feet of copper wire No. 10, B. & S. gauge, 
is approximately one ohm at 20° C. (See wire table.) 



OHM'S LAW 6 1 

3. A graphite stick 10 inches long by \ inch in diameter may have a 
resistance as high as 700,000 ohms.* 

4. A cable such as is used by electric light companies for underground 
distribution may have a resistance, when first installed, of 250 megohms ; 
a megohm is a million ohms (insulation resistance). 

5. The human body has a resistance of from 1000 to 10,000 ohms. 
Most of this resistance is due to the skin of the body. 

A large size resistance but of low value is shown in Fig. 98. 

From the above list it will be noted that no idea can be 
gained of the magnitude of a resistance from its size, color, 
or other physical characteristics. 

Circular Mil. — In resistance calculations the term cir- 
cular mil applied to the cross section of a wire, frequently 
appears. The circular inil is a small circle having a 
diameter of 07ie mil, that is, yoW ^^ ^^ inch. Care should 
be taken not to confuse the term mils with circular mils. If 
the diameter of a wire is known in thousandths of an inch, its 
cross section may be obtained 
in circular mils by squaring kREA-i^'^ 
this quantity. For instance, C^a 
a cable with a diameter of i V_^ 
inch would have one thousand «-i— > 




one-thousandths of an inch .fig. 99. - Circular Mil. 

(ioTo)^ squaring the 1000, or multiplying ( 1000 x 1000 
= 1,000,000), we obtain 1,000,000 circular mils. A one 
million circular mil cable when bare of insulation has a 
diameter of one inch. The reason why the diameter in 
mils squared gives the area in circular mils may be under- 
stood by considering the areas of two circles. Fig. 99, a and 
b, whose diameters are A and B. 

Let A = i-oVo ^^ ^^ inch, or equal to i mil, in which case 

* Convenient resistances made of graphite can be purchased from the Dixon 
Crucible Co., of New York, for about 18 cents. Their resistance varies from 1000 to 
700,000 ohms. They are small in size and are quite useful for experimental work. 



62 EXPERIMENTAL ELECTRICITY 

the area of the circle ''a'' equals the unit according to 
definition, one circular mil. The area of any circle is ex- 
pressed as being tt, 3.1416, times the diameter squared 

divided by 4 or area oi a = and area of <^ = » 

4 4 

How many times does the circular mil "a" go into the 
area *' ^ " ? Dividing one area by the other, we obtain 

4 ^7^B\ 4 ^B'' 

4 
or the areas vary as the squares of their respective diameter. 

Substituting in — , for A its value i, we obtain A'^ ox A x A 

B^ 
= 1x1 = 1, or — = B^. The area 5 is then B"^ tim^es 
I 

greater than a, which is one circular mil. The area of d in 

circular mils is then equal to its diameter B in mils squared. 

Resistance of Copper Conductors. — The resistance of 

copper wire or cables having a circular cross section may 

be readily calculated from the formula, 

lO.sS X lenp^th in feet 

resistance = "^ r^ 

cross section in circular mils 

This formula is true for copper wire at 98 % purity, Mat- 
thiessen standard at 68° F. 

This formula expresses the fact that 07ie mil foot of 
copper has a resistance of 10.35 ohms at 68"^ F. 

The sum R of resistances i\ and r^ placed in series is 
expressed as follows : 

When placed in parallel they are expressed in the follow- 
ing form : 



OHM'S LAW 63 

1 = 1 + 1 

R ;-! r2 

^1 ;.2 ^ ^(^.1 ^ ^.2^^ 



ri+r2 



Temperature Coefficient. — Most pure metals increase 
their resistance with an increase in temperature, although 
some substances, such as carbon, decrease their resistance 
with an increase of temperature. Some alloys are manu- 
factured, such as la la wire, Krupp resistance wire, Supe- 
rior wire, etc., with very low temperatttre coefficients, slight 
changes with increase in resistance. For pure copper and 
most pure metals the temperature coefficient is .0042 per 
ohm per degree centigrade above zero. If R^ be the re- 
sistance of any wire at zero, then .0042 x t x Rq would be 
the change for temperature t of the wire, and Rq{i-\- .0042 /) 
would be the resistance Rf at temperature L 
Rt=RQ(i + .0042/). 

The wire table on pages 64 and 65 shows the resistance 
of various sizes of copper wire at various temperatures. 
For the larger sizes of cables the formulae for resistance 
and temperature coefficients should be used. 

Electro-motive Force, — The electro-motive force usually 
expressed in vo/ts is the pressure which forces the elec- 
tricity through the circuit. It is sometimes compared to 
water pressure. In a dynamo electric machine the volts 
generated depend upon the number of lines of force cut in 
one second. When 100,000,000 (10^) lines of force are 
cut in one second, one zwlt is generated. If 200,000,000 
lines of force be cut in one second, two volts are generated. 
Voltage may be de fitted as the rate of cutting of lijtes of 
force. An electro-motive force or a voltage may be gen- 



64 



EXPERIMENTAL ELECTRICITY 



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OHM'S LAW 



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66 



EXPERIMENTAL ELECTRICITY 



erated by chemical action, as in a battery. These volts 
are identical with those generated by a dynamo, although 
their manner of production is different. The voltage of a 
generator is expressed by the product of the number of 
armature turns, the field flux and the speed, divided by 
10^. Varying either of these three quantities will change 
the magnitude of the electro-motive force. 
For convenience the term electro-motive 
force is referred to as e. in.f. When an 
e. m. f. is forcing a current of electricity 
through a series circuit, each ohm requires 
the same pressure. As in Fig. lOO, if one 



-wvw^ 



lis VOLTS 
d.C. + 



10 
OHMS 



K^ 



AA/WVAA- 



KZ^ 



Fig. loo. — Principle part of a circuit has twice the resistance 
of Distribution of of the othcr part of the circuit, it will have 
Potential. twicc the potential difference across its 

terminals. This leads to the conclusion : 

That the distribution of potential in a series direct current 
circtnt is always proportional to the resistance of the elements. 
Standard Cells. — There are two types of standard cells 
which are in use extensively in this 
country, that which is known as the 
Carhart-Clark cell, Fig. loi, and that 
which is known as the Weston Cad- 
mium Cell, Figs. 102 a, 102 b. At the 
International Electrical Congress, 
held in Chicago, in 1893, the follow- 
ing resolution was passed by the com- 
mittee on standards concerning the 
unit of electro-motive force : 

It was resolved that as a unit of 
electro-motive force, the international 
volt be used; this is the e. m. f. that, ^ , ^, , 

Fig, loi.— Carhart-Clark 

Steadily applied to a conductor whose ceii. 




OHM'S LAW 67 

resistance is one international ohm, will produce a current 
of one international ampere, and which is represented suf- 
ficiently well for practical use by W%\ of the e. m. f . between 




Fig. 102 a. — Weston Cell. 



the poles or electrodes of the voltaic cell known as Clark's 
cell at a temperature of 15° C. and prepared in the manner 
described in the accompanying specifications. 



6S 



EXPERIMENTAL ELECTRICITY 



The Clark cell, sold commercially as the Carhart-Clark 
cell, Fig. loi, has for its positive element mercury and its 
negative element amalgamated zinc, the electrolytes being 
a saturated solution of sulphate of zinc and mercurous 
sulphate. With the Carhart-Clark cell, the zinc sulphate 
solution is saturated at o° C, instead of at 15° C, the tem- 
perature coefficient being only half as great, and the 
e. m. f. 1.440 volts. With the Clark cell, the e. m. f. is 1.434 
international volts. The cell is provided with a small 
thermometer, its e. m. f . for any temperature being cal- 
culated from the following formula : 

£"= 1.4328 -o.ooii9(/- i5°C.)- 0.000007 (^- i5°C.).2 

The Weston Cadmium cell has as elements cadmium and 
mercury, the electrolytes being the sulphate 

of cadmium and 
mercury. 

For details of 
specification, see 
Foster's Hand- 
book, pages 10-13. 
In this cell, as 
made by Dr. Wes- 
ton, the cadmium 
sulphate solution is 
saturated at 4° C , 
resulting in prac- 
tically a zero tem- 
perature coeffi- 
cient. The e. m. f. of the cell is i. 01 985 international volts, 
and it will remain constant provided current not in excess 
of .0001 ampere is used from the cell. In other words, a 
resistance less than 10,000 ohms should never be connected 




Fig. 102 (^. — Weston Cell. 



OHM'S LAW 69 

across the terminals of the cell* The method employed by 
the various manufacturing companies in this country and 
the Reichsanstalt in Germany is to make up at one time a 
large number of these cells and check them against each 
other from time to time. The cells are usually made with 
insulated binding posts, so that in handling them the fingers 
of the operator will not come into contact with the bare 
parts of both terminals at the same time, as in this event a 
resistance of about 2000 ohms would be placed in series 
with the cell. This value, 2000 ohms, for the resistance 
of the hand is only approximate, as it is a widely variable 
quantity, depending upon the contact made and depending 
also upon the individual. The writer has found persons 
whose bodily resistance as measured from hand to hand 
varied from 1500 to 10,000 ohms. 

The standard cell is a very important instrument to use 
in connection with potentiometers for checking voltmeters 
and ammeters. Details for doing this are given in the 
chapter on Electrical Measurement. Two standard cells 
may be checked against each other by means of a poten- 
tiometer, and this is the method usually followed in practice. 
The late Professor Carhart devoted many years of his life 
to the development of the Carhart-Clark cell. 

Current. — The term anrejit of electricity is usually 
defined in terms of its effect on the circuit. A current of 
one ampere is one which : 

(i) When passed through a solution of silver chloride 
will deposit in one second .0011 181 grams of silver 
on the negative pole. 

(2) When passed through a wire bent in the form of an 
arc of one centimeter radius, of one centimeter of 

* It sometimes becomes necessary to use a resistance less than 10,000 ohms, 
in which event the circuit should be closed for only a short instant. 



70 EXPERIMENTAL ELECTRICITY 

length, will exert a force of one tenth of a dyne on a 
unit magnet pole placed at its center. The absolute 
unit of current is lo times as great, in which case the 
force would be i dyne. 

(3) When passed through a resistance of one ohm will 
develop .24 gram-calories of heat per second. 

(4) Will be forced through a resistance of one ohm by 
a pressure of one volt. 

Definition 4 is the accepted one as recommended by the 
Standardization Committee of the American Institute of 
Electrical Engineers. 

Some idea of the dimensions of the ampere may be 
gained from the following : 

One i6-candle-power carbon filament lamp takes .43 
ampere. 

One 1,000,000-circular-mil cable, one inch in diameter, 
will carry 1000 amperes without heating excessively. 

One 40-horsepower motor such as is used in a 4-motor 
trolley car may take 200 amperes at starting. 

One 8-car heavy subway train or an electric locomotive 
such as is used by the New York Central and Hudson 
River R. R. may take 3000 amperes at starting. 

An inclosed arc lamp such as is used for illuminating 
the New York City streets takes about 4.5 amperes. 
A flaming arc lamp may take as high as 12 amperes. 
A projecting lantern usually uses about 15 amperes in 
its arc. 

An incandescent lighting circuit is not supposed to 
carry more than 6 amperes per outlet. A standard 
resistance box containing low resistances should not 
have more than .4 ampere pass through any of the 
low resistances and preferably less than this. A direct 
current of .1 ampere passed through the human body 



OHM'S LAW yi 

has been known to be fatal. A telephone receiver 

can detect a current as low as of an ampere 

100,000 

by its click. 

A better conception of the idea of current can perhaps 
be gained by considering two conductors, one carrying a 
current twice that carried by the other. Suppose one is 
looking at the cross section of both of these conductors 
and that one can count at any instant of time the number 
of "electrons," or charged carriers of electricity, which are 
passing the section of both conductors under considera- 
tion, it is quite probable that one would count twice as 
many of these electrons in one case as in the other. Current 
is defined by some beginners as the quantity of electricity 
which passes through a circuit per unit of time. This 
definition is incorrect, as the quantity of electricity per 
unit of time or the ampere second is defined by the unit 
coulomb (one ampere flowing for one second is the cou- 
lomb). Current has but one dimension, a cross section as 
it were, and neither length nor time. 

Ohm's Law.* — Dr. Simon Ohm, in 1826, formulated a 
law which exhibits the relation of electro-motive force, cur- 
rent, and resistance. With a thorough grasp of the appli- 
cation of Ohm's law, any problem in direct currents may be 
readily solved. The same relation of the quantities com- 
posing Ohm's law, namely electro-motive force, resistance, 
and current, also applies in the alternating current circuit; 
but two other factors appear here and must be considered, 
namely, inductance and capacity. These factors are also 
present in the direct current circuit, but they only affect the 

* This law was first stated in his " Bestimmung des Gesetzes, nach welchem die 
Metalle die Kontaktelekti izitat leiten," 1826, and was developed and proved mathe' 
matically in his " Die galvanische Kette mathematisch bearbeitet," 1827. 



72 EXPERIMENTAL ELECTRICITY 

circuit when a change in the current flow is taking place. 
For instance, the inductive effect of a field of a direct cur- 
rent generator may be so great that one second will elapse 
after closing the circuit before the current reaches its maxi- 
mum value. This can be demonstrated by closing and 
opening the field circuit of a lo-kw. machine. If the switch 
is closed and opened quickly, only a small spark occurs. 
If the interval of its remaining closed be slightly increased, 
the arc on opening the circuit will increase. After the 
switch has been closed for several seconds and is then 
opened, the arc will be very much increased. 

Ohm's law may be expressed as folloivs : In any direct 
current circnit containing resistance, the cicrrent in amperes 
which is passing through this resistance will always be equal 
to the differe7tce of potential across this resistance, measured 
i7i volts, divided by tJie value of the resistance of tJie circidt, 
expressed in oJims. 

116 VOLTS _ Experiment 35. Take a ii6-volt circuit, 

I place two i6-candle-power lamps in series 

V^ with a 2-ampere ammeter and the circuit, Fig. 

— ' 103, and measure the difference in potential 

across each lamp with a voltmeter. Calculate 

the resistance of the lamp when the circuit is 



Fig. 103. — Ohm's Law. closed. 

If E be used to designate the difference of potential in 
volts, R to express the resistance in ohms, and / the current 
in amperes, then. 



4^ 



■=i 



For many years the letter C was used to designate cur- 
rent, but the capital / is now used exclusively for this pur- 
pose, C being used to designate capacity. A convenient 



OHM'S LAW 73 

way of remembering the relation of the quantities in Ohm's 
law is offered by the following form : 

E 



Ix R 



= I. 



If in a given circuit we know any two of these quantities, 
we can find the third from the relation mentioned above. 
For instance, if we wish to know the resistance R of the 
circuit, and we know the electro-motive force E and the cur- 
rent /, place the finger over the R in the above formula, 

and we have the result R = ^ . To obtain the value of the 

ele'ctro-motive force when the current and resistance are 
known, place the finger over the E, and we obtain E =1 x R. 
The practical interpretation of Ohm's law is that when a 
circuit is once formed, the electro-motive force always sends 
a maximum of current through the circuit. When the cur- 
rent is passing through the circuit, each ohm requires the 
same pressure in volts to force the current through it. If 
we had a resistance of lOO ohms connected across a circuit 
of 1 16 volts, we would have a current of \^^ = 1.16 amperes 
passing through the resistance, and each ohm would re- 
quire 1. 16 volts to force this current through it. If we 
should measure part of such a resistance, with a voltmeter 
so as to include 50 ohms, it would indicate on the voltmeter 
58 volts. An interesting consequence of such an experi- 
ment is that t/ie distribiUion of potential is proportional to 
the resistance. If two unequal resistances, such as 20 
ohms and 100 ohms, be placed in series with a 120- volt 
circuit, the potential will distribute itself proportionally to 
their respective resistances, 20 volts being the difference 
of potential across the 20 ohms, and 100 volts the potential 
difference across the 100 ohms, or 



74 EXPERIMENTAL ELECTRICITY 

e:e' ::R'.R', 
20: 100 : : 20 : lOO, 

where e and e^ are the two e. m. f's and R and R' are their 
respective resistances. 

An interesting case of the appUcation of this principle 
of the distribution of potential in a circuit is that of the 
Brush direct current series arc circuit, each circuit contain- 
ing 50 lamps of 40 volts difference of potential, connected 
in series, and making a total potential of 2000 volts. There 
are three of these 2000-volt circuits to a machine which 
may be connected in series, making 6000 volts. Although 
there is only 40 volts difference of potential to each lamp, 
yet if the circuit is opened at any lamp, the potential across 
the gap rises to 6000 volts, the resistance of the air gap 
being greater than that of the entire circuit. Some idea of 
the resistance of an air gap may be obtained when it is 
stated that it requires 20,000 volts to jump a one-inch air 
gap. Where there is danger of coming into contact with a 
high voltage circuit, it must be realized that quite possibly 
the circuit may be partially grounded at one or m.ore points. 
This is a condition hkely to occur especially with overhead 
wires, whose insulation deteriorates quite rapidly the first 
year after installation. Suppose a person to be standing 
on the ground touching a live wire which had sagged. 
Suppose also the circuit to be grounded at some other point. 
The difference of potential across the complete grounded 
circuit including the person would be equal to the sum of 
the differences of potential of the lamps included between 
the two points. 

Example. — If a series arc circuit of 40 volts per lamp were grounded 
at a certain point, and a person whose bodily resistance was 8000 ohms 



d.c. 



OHM'S LAW 75 

making contact, 20 lamps from the point where the line was grounded, 
how much current would pass through his body ? Attswer : approxi- 
mately jV ampere. 

One reason why it is not desirable to operate an ordi- 
nary call bell on a I i6-volt circuit with a lamp in series to 
cut down the current is that every time the armature of 
the bell moves over, opening the circuit, an arc is produced 
which corresponds to a ii6-volt circuit. + ne volts 
The magnitude of the arc is always pro- 
portional to the potential of the circuit. 
It is possible to open a direct current 
cii:cuit of 200 amperes and 2 volts with a 
very small switch, whereas if the voltage 
were 500 volts and the current 10 amperes, 
a much heavier arc would be formed. contact) 

Where it is desirable to operate a low I .___'^ 

voltage device, such as a motor or an fig. 104. Method of 
electric bell, from a ii6-volt circuit, a re- obtaining Low Voltage. 
sistance may be connected across the service, and part of 
this resistance may be shunted off to the consuming device, 
as in Fig. 104. 

QUESTIONS 

1. A carbon filament lamp consumes 3.1 watts per candle power; a 
tantalum lamp, 2 watts per candle power; and a tungsten lamp, 1.25 
watts per candle power. What would be the wattage and current con- 
sumption of these lamps if of 16 candle and burned on a ii8-volt 
circuit? Answer: watts, 49.6, 32, 20.0 ; ampere, .42, .27, .17. 

2. An arc lamp used in a projecting lantern on a ii8-volt circuit, has 
a potential of 40 volts across the arc and 15 amperes passing through the 
lamp. What is the value in ohms of the resistance placed in series 
with the arc ? Answer : 5.2 ohms. If the resistance of the arc changes 
so that the current falls to 10 amperes, what will be the potential across 
the resistance ? Answer : 52 volts. 

3. In a series arc circuit of 6000 volts, containing 50 volts to the lamp, 
the line becomes grounded at a certain point, making a ground whose 



76 EXPERIMENTAL ELECTRICITY 

resistance is looo ohms. A man having a bodily resistance of 5000 
ohms grounded makes contact with the arc circuit, 60 lamps from the 
grounded point. What current will pass through his body, assuming 
his contact resistance is practically zero. Answer : .5 ampere, which 
would be sufficient to kill if the contact were sufficiently long. 

4. A dry battery having a normal voltage of i .5 volts is short-circuited 
on an ammeter having a negligible resistance. The ammeter indicates 
10 amperes. What is the internal resistance of the battery ? Answer : 
.15 ohms. 

5. If a storage battery of 2 volts having an internal resistance of .01 
ohms be short-circuited, what circuit will flow? Answer : 200 amperes. 

6. A dynamo generating 120 volts, uncompounded, has its voltage 
fall to no volts when supplying 100 amperes of load. What is the 
equivalent resistance of the armature circuit? Ansiuer : o.io ohm. 



CHAPTER V 

PRIMARY AND STORAGE BATTERIES 

In 1786 Galvani, a physician of Bologna, noticed that, 
when he suspended a pair of frog's legs on a window hook, 
the legs twitched. He supposed that an electric current 
was generated by the frog's legs ; whereas the twitching, 
as noticed later by Volta (a professor in the University of 
Pavia), was due to the two dissimilar metals, such as an 
iron hook and a copper window frame, being brought into 
contact, and the nerves of the frog's legs being excited by 
the electricity generated at the junction. 

• ^ ^, „ . ^ Fig. 105. — Galvani's 

Experiment 36. Place a small piece of meat Experiment. 

in contact between two dissimilar metals such as 

copper and zinc, and connect the two metals to a sensitive galvanom- 
eter, Fig. 105. Note deflection of the galvanometer. 

The Simple Cell. — The simple cell, consisting of two dis- 
similar metals placed in an electrolyte, was first developed 

by Volta. When the terminals of such a 

/ *^^ N^ _j. cell are brought into contact, an electric 
current flows through the circuit, as indi- 
cated in Fig. 106. 



'tfh 



Zri 



^ 



Cu 



Experiment 37. Place a strip of zinc and a piece 
of copper, Fig. 106, in a vessel containing dilute sul- 
FiG. 106. — Voltaic phuric acid (i part of acid to 20 of water). Notice 
Cell. that, if the zinc is not amalgamated^ or coated 

77 



78 EXPERIMENTAL ELECTRICITY 

with mercury, the zinc slowly dissolves into solution. Make a contact 
between the copper and the zinc electrodes, and notice that bubbles be- 
gin to rise from the copper electrode. Open the circuit, and notice that 
the bubbles cease to rise from the copper plate. Such a cell is termed a 
voltaic cell. 

On connecting the terminals of such a cell to a voltmeter, 
it will be noticed that the copper is positive and the zinc 
terminal negative ^ the potential of the cell being about 1.05 
volts. When the circuit is closed, the current passes in 
the external circuit from copper to zinc and in the solution 
from zinc to copper, forming a circuit. The metal plates 
suspended in the liquid are termed electrodes^ and the 
conducting solution is termed an electrolyte. 

Note. An experimental tank, Fig. 107, for projecting purposes to 
i qI" I I ' o 7 be used on a horizontal lantern can be made by taking 



two pieces of x f" plate glass, 4'' x 5'', and separating 

'1 "^^ 1° ' them by a piece of rubber gas tubing bent in the form of a 

^ U. The tank may be held together by two clamps made 

^ from thin cedar, held together by 8-32 machine screws. 



•n — 

Fig. 107.— The advantage of a simple tank of this kind is that it may 
Projecting be readily taken apart and cleaned, and that liquids can 
Tank. remain in it for some time. Such a tank, indeed, is supe- 

rior in every way, except perhaps in appearance, to one that is ce- 
mented. 

Chemical Action of Cell. — When the simple cell is in 
operation, the current, passing through the sulphuric acid 
solution, H2SO4, gradually electrolyzes it, splitting it up 
into its components, H2, or two parts of Jiydrogen, and 
SO4, or sidphion. The SO4 has a very strong affinity for 
zinc, combining with it and forming ZnSO^, or zinc sul- 
phate, leaving free the hydrogen gas, which rises at the 
copper electrode. The reaction is expressed as follows : 

Zn + H2SO4 = ZnSO^ + H2. 



PRIMARY AND STORAGE BATTERIES 



A number of cells placed in series, the positive of one cell 
connected to the negative of the other cell, form a battery. 
Volta was one of the first to discover the cumulative action. 
The E. M. F. of Cells. — Any two dissimilar metals im- 
mersed in an acid solution constitute a cell. The poten- 
tial, or electro-motive force, of such a cell is independent of 
the size of the electrodes. Changing the electrolyte of the 
of the cell will affect the electro-motive force, and chang- 
ing the character of the electrodes will also affect the 
potential. The metals arrange themselves in a definite 
series : — 

Zinc 

Iron 

Tin 

Lead 

Copper 

Silver 

Platinum 

Carbon 



Effect of changing Electrodes. 

Experiment 38. Connect a projecting galvanometer, having a low 
resistance in series with it so as to make a voltmeter of it, to a wooden 
support in which are two holes containing mercury, 
Fig. 108, into which the galvanometer wires dip. 
Have a number of various electrodes, 
such as zinc, iron, tin, lead, copper, 
platinum, and carbon, Fig. 109, which 
have curved terminals that can fit also 
in the mercury cups. Mount two 
electrodes, copper and zinc, in a 
tank, Fig. 109, containing dilute sulphuric acid. Place 
various combinations of electrodes in the tank, and 
notice magnitude and direction of deflection. Zinc 
and lead will cause a deflection in one direction, lead 





Fig. 108. — Sup- 
port for Elec- 
trodes. 



Fig. 109. — Sup- 
port in Posi- 
tion. 



80 EXPERIMENTAL ELECTRICITY 

being positive to the zinc. Copper substituted for the zinc will cause 
a deflection in the opposite direction, copper being positive to the lead. 
Substitute zinc for the lead, and the deflection v^^ill be in the same direc- 
tion, but still greater. Carbon and zinc aff"ord about the greatest deflec- 
tion and are used extensively in practice, as both are comparatively 
cheap. The Mesco dry battery, the Leclanche cell, the Grenet cell, and 
the Bunsen cell all employ this combination of elements. 

Effect of changing Electrolytes. 

Experiment 39. Repeat the previous experiment, using the same elec- 
trodes, such as zinc and copper, but change the electrolyte, using instead 
a solution of salt, of soapsuds, etc. The e. m. f. values vv^ill change for 
the different solutions. 

From the two preceding experiments it would seem that 
a suitable cell could be made from any two dissimilar 
metals immersed in any electrolyte. While it is true that 
a cell could be produced in this manner, it would not be 
suitable for commercial work, owing to various transforma- 
tions which occur in the cell while operating. This matter 
will be discussed later. Electrolytes, in the first place, 
must be acid solutions, so that they will attack one elec- 
trode more readily than the other; they further must not 
disintegrate in themselves. When bubbles of hydrogen 
gas are developed in the cell, they form a coating on one 
of the electrodes which tends to insulate the cell and also 

to change its polarity. This effect is termed 

polarization. 

Polarization. 

Experiment 40. Place a zinc and copper electrode 

Fig. iio.- Polar- -^^ ^ solution of H.SO^ so that but a small part of 

the electrodes is immersed ; connect the terminals of 

the electrodes to an electric bell. Notice that after a time the intensity 

of ring of the bell decreases, until finally the bell armature ceases to 



PRIMARY AND STORAGE BATTERIES 




move. Heat the electrodes over a bunsen burner, and repeat the ex- 
periment, showing the effects of polarization. The heat of the flame 
will drive off the polarizing gas. 

Experiment 41. Repeat the experiment, substituting a low-range 
voltmeter for the bell, and notice that the deflection of the voltmeter 
will be. great at first and will then gradually fall as the battery polarizes. 

Depolarizers. — Various substances are used as depolari- 
zers in batteries, such as manganese in the Leclanche cell, 
and bromide of potash in the Grenet cell. 
In the carbon cell. Fig. iii, the carbon 
electrode has a large surface to decrease 
the polarizing effect. The action of the de- 
polarizing element is to absorb the hydrogen 
gas developed at the positive electrode and 
so prevent this polarizing action. What 
might be termed chemical depolarizers, like 
manganese, can only be employed for cells fig. 
upon intermittent work, such as call bells. 
For continuous duty or closed circuit work, such as fire alarm 
systems, telegraph systems, etc., another method of de- 
polarizing is employed in which two fluids are used in the 
cell, the principle of electrolysis being utilized. 

Experiment 42. Fill a large-sized vessel full of water. Add a few 
drops of permanganate of potash solution, coloring the water a deep 
wine color. Dissolve in another vessel a few crystals of " hypo,'' used 
for photographic work. Add a few drops of the hypo solution to the 
colored potash solution, stirring the liquid with a glass rod. The ac- 
tion of the permanganate solution is so strong that the liquid will 
quickly lose its red color and become clear. This illustrates the action 
of a depolarizer. If the liquid becomes milky instead of clear adc| a 
few drops of H2SO4 beforehand. 

Electrolytic Depolarizers. — A two-fluid cell contains two 
electrolytes separated either by gravity or by a porous cup, 
the electrodes projecting each in the separate solutions. 



III. — Carbon 
Cell. 



82 EXPERIMENTAL ELECTRICITY 

The Daniell cell, the gravity cell, and the Grove cells are 
of this type. With the Daniell cell a zinc electrode pro- 
jects in a porous cup containing a dilute solution of sul- 
phuric acid, H2SO4, and a copper electrode projects in a 
saturated solution of copper sulphate, CuSO^. When the 
cell is in action the zinc has a strong affinity for the acid 
radical SO4 in the H2SO4, combining with it, and leaving 
two molecules of hydrogen, Hg, forming zinc sulphate, 
ZnS04. The free hydrogen, H2, has a strong affinity for 
the SO4 of the CuSO^ solution, forming H2SO4, leaving the 
free copper, which is deposited electrolytically upon the 
copper electrode. The reaction may be written as follows : 

Zn + H2SO4 = ZnS04 + H2. 
H2 + CUSO4 = H2SO4 + Cu. 

Electrolytic Condensers. — An interesting use is made 

of the effects of polarization in a device termed an elec- 

115V0UTS trolytic condenser, Fig. 112. The electro- 

d c 

J — + ^ r lytic condenser consists of two ordinary lead 
"jj~jr plates immersed in a vessel containing dilute 

'- ■ sulphuric acid. The two electrodes are con- 

nected up through a suitable resistance, such 
as a 50-candle-power lamp, to a source of con- 
troiytic Con- stant potential, such as a ii6-volt direct cur- 
denser, rent Edison service. When so connected, the 
cell will yield a potential of about 2 volts. Terminals may 
be led off from the battery when so con- + ns volts ^ ^ 

-^ I rlc. (^ I 

nected to a bell circuit, a plating bath cir- rj-f^ j=l-fj 
cuit, or any other apparatus where a low 11 II 
constant voltage is required. An electro- fig. 113, — Electrolytic 
lytic condenser circuit is preferable to the Ceiis in Series. 
1 1 6- volt circuit with resistance in series. This eliminates 
the arc which is produced when the 11 6- volt circuit is 



PRIMARY AND STORAGE BATTERIES 83 

opened. The arc formed for the same amperage varies 
with the potential of the circuit. A number of cells may 
be placed in series, Fig. 113, yielding any potential desired. 
This apparatus is quite economical in energy consumption. 

Experiment 43. Take two lead strips and suspend in a dish of dilute 
sulphuric acid, and then charge for an instant through a i6-candle-power 
lamp in series with a ii6-volt direct current circuit. Open the circuit 
and connect the terminals of plates to a galvanometer, noting the kick. 

Experiment 44. Connect an electrolytic tank in series with a 16- 
candle-power lamp and a ii6-volt circuit, and shunt off from the con- 
denser to an electric bell. Change i6-candle-power lamp to 32-candle- 
power, and to 50-candle-power, noting the effect on the bell. 

When an electric current passes through an electrolytic 
condenser, it decomposes the liquid, forming gas at the 
electrodes, oxygen and hydrogen resulting. These two 
gases, being formed at the electrodes, create the potential. 

Closed Circuit Cells. — Cells intended for continuous 
operation, such as fire alarm and telegraph systems, are 
termed closed circuit cells. The termi battery is frequently 
used in speaking of a single cell, but this usage is improper, 
as a battery implies a number of cells. Closed circuit 
cells are usually of the two-fluid type, have a high internal 
resistance, give a small current on short circuit, and when 
once set up must have a constant load, whether the load 
be real or artificial. If the load on the cell be removed, 
the liquids are likely to mix. 

Daniell Cell. — This cell, as stated on page 82, consists 
of a porous cup containing a zinc electrode immersed in a 
dilute sulphuric acid solution. The porous cup is sus- 
pended in a vessel containing a saturated solution of copper 
sulphate surrounded with a copper electrode. After a 
time it is necessary to replenish the hquid, as the copper 
sulphate becomes weak as a result of having had the 



84 - EXPERIMENTAL- ELECTRICITY 

metallic copper taken from it. The sulphuric acid also 
becomes zinc sulphate. The cell has a remarkably con- 
stant potential of 1.178 volts with i in 12 acid and is 
used by some investigators as a sub-standard cell. 

Experiment 45. Set up a Daniell cell, taking care to amalgamate the 
zinc. 

Local Action. — Most commercial zinc contains impuri- 
ties such as iron. When a zinc electrode is suspended in 
a cell, the iron and the zinc in contact form a small short- 
circuited cell which wastes the zinc away. To prevent 
this from taking place, a phenomenon termed local action-, 
the zinc should be amalgamated by first dipping it in acid 
and then allowing it to come into contact with mercury, 
forming a film of mercury over the zinc. The mercury 
forms an amalgam with the zinc, which floats the lighter 
impurities such as iron to the surface, separating the iron 
from the zinc, so that both are not in contact with the acid 
at the same time. Local action is not alone confined to 
batteries, but occurs whenever two dissimilar metals in con- 
tact are exposed to the elements, such as salt air or salt 
water. Thus, when in street manholes that contain feeders 
supported on iron straps fastened to racks, salt water has 
entered the manholes, the lead armor of the sheathing of 
the cable has often been found to be eaten away where 
the cable had come into contact with the supporting iron 
strips. In aerial transmission lines where aluminium wire 
is used, it is necessary to fasten the wires together 
with aluminium connectors, for where the wires are 
joined together with copper connectors, it has been found 
that electrolytic action frequently occurs. On the bottom 
of boats where iron bolts have come into contact with the 
copper sheathing, electrolytic action has been noticed. It 



PRIMARY AND STORAGE BATTERIES 



85 




is always well to remember that two dissimilar metals in 
contact exposed to an acid or a salt solution constitute a 
cell, and electrolytic action, sometimes called galvanic 
action, is likely to occur. 

Gravity Cell. — The gravity cell, Fig. 1 14, is a special 
form of Daniell cell used extensively in practice. The 
copper electrode, Fig. 115, is fan-shaped and is placed in 
the bottom of the jar sub- 
merged in a concentrated 
solution of copper sulphate. 
Over the top of the copper 
electrode is sometimes 
placed a copper tray con- 
taining holes. Over the fi°- xi4.- Gravity cdi (m. e. s. Co.). 

tray is placed a layer of copper sulphate crystals. This 
method places the crystals in such a position that they 
will not interfere with the contact of the 
electrolyte against the copper electrode; 
it also serves with the assistance of gravity 
to keep the copper sulphate saturated. 
In filling the cell over the top of the cop- 
per sulphate crystals is placed a round thin 
board which floats on top of the copper 
sulphate solution. A di- 
lute solution of sulphuric acid is next poured 
through a funnel so that the stream will 
strike the center of the float. The float 
will gradually rise until the vessel is almost 
full of liquid. The zinc electrode, Fig. 116, fig. 116. — Crow- 

- , , . 1 1, foot Zinc. 

IS next mtroduced, completmg the cell. 
Occasionally the zinc electrode has a central support 
which passes through a porcelain cover, preventing evapo- 
ration. 




Fig. 115. — Copper 
Electrode. 




S6 EXPERIMENTAL ELECTRICITY 



Experiment 46. Form a circuit of a 16- 
-•+- candle-power lamp, a copper and a carbon 

^^^dT^^ electrode immersed in copper sulphate solu- 

I CARBON \\.on, and a ii6-volt direct current source of 
supply. Fig. 117. Be sure that the carbon rod 
is connected to the negative electrode. Turn 
on the current and notice that after a time 
copper is deposited upon the carbon rod. This 
-^ experiment illustrates the action taking place 

'^^l^~ ^ ^"S o \^ a gravity cell, and shows that the copper 
Copper. 11. . . , , . \ 

electrode is positive and that copper is de- 
posited upon it while the cell is in action. 

Bunsen and Grove Cells. — Both of these cells are of the 
two-fluid type. The Bunsen cell has a zinc electrode im- 
mersed in dilute H2SO and the carbon suspended in a 
porous cup containing nitric acid. The Grove cell is some- 
what similar in detail, except that a platinum electrode is 
substituted for the carbon electrode in the porous cup. 
These cells give potentials of 1.75 to 1.95 volts and pro- 
duce constant currents of considerable strength. They are 
especially adapted for experimental work, the only objec- 
tion to them being the noxious fumes of the nitric acid. 
This difficulty, however, can be overcome by using a 
solution of bichromate of potash. 

Grenet Cell. — The Grenet cell consists of two carbon 
plates and a zinc plate immersed in a solution of bichromate 
of potash. When not in use, the zinc should be raised. 
This cell gives 2 volts, a very high potential compared with 
other cells, and is well adapted for experimental work, 
especially in cases where the building in which the experi- 
menting is done is not wired for electric current. Many 
of the experiments described in this text can be performed 
with three of these cells in series, giving 6 volts. The 
Grenet cell after a time loses its strength, owing to the 



PRIMARY AND STORAGE BATTERIES 



87 



depolarizing action of the electrolyte. The liquid loses its 
deep red color with age. 

Open Circuit Cells. — Cells which are used infrequently 
as in the case of bells and annunciators, are termed open 
circuit cells. Such cells as a rule give a high amperage on 
short circuit. Some forms of dry battery, like the Red 
Seal Dry Battery, Fig. 121, made by the Manhattan Elec- 
trical Supply Co. of New York, will yield 20 amperes on 
short circuit, having a potential of 1.5 volts. Such a cell 
has naturally a low internal resistance, 1.5/20 =.075 ohms. 
Cells having high amperages have a slightly decreased 
life. The above-mentioned cell has a life of one year. 

Leclanche Cell. — This cell. Fig. 1 1 8, contains a carbon 
electrode surrounded with broken carbon and dioxide of 
manganese suspended in a porous cup sealed 
at the top and having a small air vent. The 
porous cup is placed in a jar 
containing sal ammoniac solu- 
tion together with the zinc 
electrode. Fig. 119. The top 
of the jar is covered with wax 
to prevent the sal ammoniac 
JIIJI solution from crawling up from 

capillary attraction and crystal- 
lizing on the top of the jar. 

Fig. 119. — Zinc ^ ^ •* 

for Leclanche The c. m. f. of this ccll is about 1.48 volts, and 
^^^^' its internal resistance is about 4 ohms. Under 

normal conditions of external use the depolarizer will take 
care of the hydrogen Hberated, but if an attempt is made to 
use this cell upon continuous service, the hydrogen gas will 
be liberated more quickly than it can be absorbed, and 
the cell will polarize. If the zinc is amalgamated, it will 
not be consumed except when the battery is in operation. 




Fig, 118. — Le- 
clanche Cell. 



88 



EXPERIMENTAL ELECTRIC/TV 



Experiment 47. — A simple way to make a saturated solution of cop- 
per sulphate to be used in connection with gravity cells is to fill a dish 
with water, place a piece of gauze in the top of the dish, and put some 
copper sulphate crystals in the gauze tray. The copper sulphate will 
dissolve and fall to the bottom of the dish in hair-like streams. This 
process will continue until the copper sulphate solution is completely 
saturated ; thereafter no more copper sulphate will dissolve. This ex- 
periment may be performed on a horizontal lantern with one of the 
small tanks shown in Fig. 107. It forms a very attrac- 
tive experiment. 

Experiment 48. — In a gravity cell the two liquids 
in contact, the copper sulphate solution and the sul- 
phuric acid, remain separate because of their difference 
in specific gravity. Take a small projecting tank and 
fill it half full of dilute sulphuric acid. Into the tank 
place a small thistle tube, allowing it to reach to the 
bottom. Slowly pour the copper sulphate solution into 
the tube, and notice the sulphuric 
acid solution rise in the tank 
while the copper sulphate solu- 
tion remains at the bottom. 



Fig 




ak...: •■:::..• 





:DRYBATTERV 



£P^ 



Dry Cells. — In recent 
Cell. years so-called dry cells, 

Fig. 120, have been placed upon the 
market, convenient to use because of 
their absence of liquid. They employ 
zinc and carbon as electrodes, the active 
mixture consisting of a combination in 
dust-like form of zinc oxide i part, sal 
ammoniac i part, chloride of zinc i part, 
water 2 parts, and plaster 3 parts. These 
cells can be purchased in quantities from 
concerns like the Manhattan Electrical 
Supply Co. of New York, for about 12^ 
cents apiece. They may be readily placed in series to ob- 
tain potentials of 100 volts or more. They cost slightly 




PRIMARY AND STORAGE BATTERIES 89 

more than the Leclanche cells to maintain, as their Hfe is 
somewhat shorter, only about one year ; but the difference 
is so slight, considering the time necessary to recharge the 
Leclanche cells, that the writer uses them exclusively. 
Their e. m. f. varies from 1.2 to 1.5 volts, and they may be 
used in calibrating voltmeters and in many forms of ex- 
perimental work. In such cases the voltage is so constant 
that they take the place of storage batteries. 



THE STORAGE BATTERY 

Theory of the Storage Battery. — The storage battery 
owes its creation to Gaston Plante, who, in i860, developed 
the first storage battery with what is known as formed 
plates. The electrodes in the Plante cell were first formed 
by charging a pair of lead plates in one direction and then 
in the opposite direction. This process of charging and 
discharging was continued for several minutes, at the end of 
which time the cathode plate became sponge lead and the 
anode plate peroxide of lead. In 1881 Emil Faure de- 
veloped an improved form of battery, using the same 
elements as Plante — peroxide of lead and sponge lead — 
except that the plates were what are termed pasted plates, 
the active material, peroxide of lead, being pasted upon the 
plates and then charged once instead of being formed elec- 
trically, as in the Plante process. Battery manufacturers at 
the present day tend to employ both processes, using the 
pasted negative plate almost exclusively, and the pasted posi- 
tive plate where a light battery is required and the service 
is gradual. For heavy station work, where the changes in 
load are sudden and where weight is not so important a 
factor, the formed Plante plate is preferred. The chemical 



90 



EXPERIMENTAL ELECTRICITY 



action which occurs in a storage battery under conditions 
of charge and discharge is expressed in the following con- 
venient way by Lamar Lyndon : 



Pb02 + H2S04- 
Pb + H2S04 = 



^ Discharge. 

PbSO^ + H2O + O 
PbS04+H2 



3 = 1 + 2 = PbOa + Pb + 2 H2SO4 = 2 PbSO^ 

-< Charge 

Interpretation, — 

Pb02 = lead peroxide 
H2SO4 = sulphuric acid 

Pb = sponge lead 

• O = oxygen 

H = hydrogen 
PbS04 = lead sulphate. 



2H2O 



Both the lead peroxide and the sponge lead are con- 
ductors of electricity, whereas the lead sulphate is an insu- 
lator. As the battery discharges, this lead 
sulphate forms on both the positive and 
negative plates, increasing the internal re- 
sistance of the cell and lowering the cell's 
voltage. As the lead sulphate is formed, 
it robs the battery solution more and more 
of its acid radical, SO., which increases the 
^ i S resistance of the conducting: solution. It is 

^ m ^ 

H p 5 interesting to note in this connection that 

33 CO H 

? S pure water has a very high resistance, as 

o 5 has also concentrated sulphuric acid, but 

any mixture of the two produces a de- 

^''';'p~y^"^''°'J creased resistance. This effect is shown 

of Resistance of 

H2SO4. in the curve, Fig. 122. 




PRIMARY AND STORAGE BATTERIES 91 

Experiment 49. Take a dish containing distilled water, arrange in 
it two platinum electrodes which have been previously washed in dis- 
tilled water, and place the electrodes in series with a 
i6-candle-powTr lamp and the i i6-volt Edison direct ' — 



•+ 



current service, Fig. 123. Turn on the power, and 
notice that the lamp will not light. This shows 
that no current of any appreciable extent passes, 
owing to the high resistance of the distilled water. F^*^- 123.— High Re- 
Add a few drops of sulphuric acid, and notice that \va^er^*^ 
the filament of the lamp becomes a dull red, show- 
ing the passage of a current. Add a few more drops of acid and notice 
that the lamp lights to practically its full intensity. 

Experiment 50. Place some lead peroxide, PbOg, on a metal plate- 
farming the positive terminal of a i i6-volt direct current service. Upon 
the top of the lead peroxide place another 
metal plate, so that the two plates will be 



^ 



16 VOLTS 



^^°4 i ~l (n) • °' separated by a layer of lead peroxide. Con- 

FiG. 124. — Conductivity of nect the second plate to the other terminal 

Pb02 and PbS04 Shown. of the direct current service, placing a 16- 
candle-power lamp in series with it. Turn on the power and notice 
that the lamp will light, owing to the conductivity of the lead peroxide. 
Repeat the previous experiment, using some sponge lead in place of the 
lead peroxide, and notice that the lamp will also light, owing to the 
conductivity of the sponge lead. 

Experiment 51. Again repeat the experiment, using lead sulphate, 
PbSO^, in place of the sponge lead, and notice that the lainp will not 
light ^ owing to high resistance of lead sulphate. (See Fig. 124.) 



Operation of a Storage Battery. — The potential of a 
lead storage cell should never be allowed to fall below 1.7 
volts, since it will become so badly sulphated that it will 
be almost impossible to restore the cell to its normal con- 
dition. The life of a battery plate under proper care is 
directly proportional to the number of times it is charged 
and discharged. Formerly, with large central stations 
such as are used by the lighting companies, it was custom- 
ary to charge the cells during light load and to discharge 



92 EXPERIMENTAL ELECTRICITY 

them during peak load. After a trial of seven years it 
was found that the plates had become practically useless. 
At present, the chief function of these batteries is to serve 
as a factor of reliability, floating continuously on the system 
to carry the load in case the generating apparatus becomes 
disabled. They also serve to regulate the unbalance on the 
three-wire system, but this factor is largely taken care of by 
balances operating in parallel with the cells. For a short 
interval of time a storage battery can carry very heavy over- 
loads, 200 and 300 % of normal. During a complete shut- 
down of one of the large generating stations in New York 
City at one time the batteries carried the entire load for 
15 minutes till the generating apparatus could be put back 
again on the system. One of the large railway companies 
in New York City has installed a storage battery through- 
out its electric system of such magnitude that, should the 
generating apparatus become disabled, the battery will 
carry the load for 40 minutes, a very valuable feature in 
rush hours. The factor of reliability or of load continuity 
has such an important bearing upon the life of the elec- 
trical industry that it is the chief asset of the storage 
battery. 

Rating of Cells. — A cell is usually rated on the basis of 
a certain current discharge for a certain number of hours. 
Thus, a lOO-ampere 8-hour cell means a continu- 
ous discharge of 100 amperes for 8 hours. If this 
rate of discharge be increased, the hour rating will 
Fig. 125. decrease. For instance, it would not be possible 
Battery ^.o maintain double this rate for 4 hours. This is 

Pellet 

due to the fact that the lead sulphate is formed in 
a layer on the outside of the pellets of the cell, as in 
Fig. 125, with a heavy rate of discharge, whereas with a 
slow rate of discharge the sulphate has time to work its 




PRIMARY AND STORAGE BATTERIES 



93 



way through the whole pellet. In the former case the 
internal resistance of the cell increases rapidly, causing the 
potential to fall. 




Experiment 52. Take a cell and discharge it at various rates, charg- 
ing after each discharge. Maintain the 
current constant while tlie discliarge is 
taking place by varying the resistance in 
series with the cell. Plot the curve, Fig. 
126, for different .discharges, using am- 
pere rate for each curve, plotting the 
curves in terms of potential and hours. 

Experiment 53. Take a storage bat- 
tery plate, Fig. 127, which has been badly 
sulphated and connect the grid of the 
plate through a i6-candle-power lamp to 
the positive terminal of a ii6-volt direct current lighting service. 

With a test wire connected to the negative 
terminal touch the plate at various points 
on the sulphated pellets, and notice that 
the lamp will not light. Touch the lead 
grid, and notice that the lamp will light. 



Fig. 126. 



AMPERE HOUR 
DISCHARGE 

— Battery Discharge 
Curve. 



r 



^smH^'- 



Fig. 127. 



- Testing Sulphated 
Plate. 



I 
t 



41' 



'"1 



How to charge a Storage Battery. — A number of storage 
cells or pairs of plates connected in series constitute a 
storage battery. For each cell a potential ^^ ^^ 
of about 2 volts is obtained. For a three- 
wire 120-240-volt circuit it requires 60 cells 
on each side of the system. In order to 
distribute 120 volts it is necessary to have 
a greater number of cells than 60 on each 
side of the system to allow for drop in volt- 
age due to resistance of distributing mains. 
A number of extra cells are therefore added to allow for 
this loss in potential, these cells being termed ejid cells, 
Fig. 128. An adjustable contact, motor-operated, serves to 



•+ + - 

Fig. 128. — End 
Cells. 



94 



EXPERIMENTAL ELECTRICITY 



6 VOLTS ,_ 





cut in the number of cells required. To charge a battery 
the current should be sent into the battery in the same 
direction in which it comes out. The posi- 
tive terminal of the battery should, in other 
words, be connected to the 
positive terminal of the charg- 
Ficx. 129. — Battery ing scrvice, Figs. 129, 131, 

Charging Circuit. ^^^ ^^^^ ^^^^^^ ^^ ^^^^^ 

to see that this connection is properly made, 

or the battery will be likely to be ruined 

beyond repair. There is no known satisfac- 
tory method of restoring a storage battery 

plate which has been badly sulphated. 

From a source of direct potential an 

ammeter should be placed in series with an 

adjustable resistance and the negative plate 

of the battery, Fig. 129. A voltmeter should 

be connected across the terminals of the 

battery, and a hydrometer should be placed in the liquid 

of the cell to note when 
the specific gravity of 
the cell is normal. A 
test hydrometer similar 
to that developed by the 
Electric Storage Battery 
Co., Fig. 130, may be 
used. The voltage of 
the cell when charged 
should be slightly above 
normal, and the specific 
gravity of the cell should 
be 1.23 Baume. When 
a battery is completely 



Fig. 130. — Test 
Hydrometer 
(E. S. B. Co.). 




Fig. 131. — Charging Circuit. 



PRIMAR Y AND STORA GE BA TTERIES 



95 



charged, it gases badly. Battery charging circuits employ- 
ing lamps and adjustable resistances are shown in Figs. 
131, 132. 




Fi(J. 132. — Charging Circuit. 

Edison Cell. — With all forms of lead storage cells there 
are certain objectionable features, such as acid fumes, the de- 
terioration of the plates, the necessity for proper ventilation, 
and the need of a fume proof room to inclose the battery. 
When storage batteries are used in automobiles, it is found 
that in time the acid fumes eat the body from out of the 
carriage. A greater objection still for automobile use is 
the excessive weight of the lead cells, the weight per pound 
of battery per watt hour of cell being high. Mr. Thomas A. 
Edison in his new form of cell has endeavored to overcome 
these difficulties. His cell consists of electrodes of iron 
and nickel made up in the form of pellets inclosed in sup- 
porting rectangular baskets of steel netting. These baskets 
are fastened into a steel grid, in which are rectangular 
openings. For an electrolyte caustic potash is used. The 



96 



EXPERIMENTAL ELECTRICITY 



result is a cell which is light and mechanically strong, with 
an electrolyte in which you could put your hands without 
injury, and yielding a large wattage per pound of cell. The 
potential of the cell is about one half that of the lead cell. 
In the early development of the cell the active material 
did not make proper contact in its inclosing baskets, an 
insufficient surface of active material was exposed, and 
gas formed between the baskets and the active material. 
It is now claimed by the manufacturers of this cell that 
practically all of these difficulties have been obviated. 

Types of Commercial Cells. — At the time that Emil 
Faure was developing the pasted type of storage battery 
plate, Mr. Charles F. Brush in 
America discovered the same prin- 
ciple, and conse- 
quently the Brush 
patents have ever 
since controlled the 
pasted type of plate 
in this country. 
The original pasted 
plate, as developed 
by Faure, consisted 
of sheets of thin lead roughened upon their 
surface. Over this surface was spread the 
lead oxide. This method, however, did not 
prove commercially successful, as the active 
material, or lead peroxide, on the positive 
plate, seemed to lose its grip upon the sup- 
porting grid and fall away. Many forms 
of grid for locking the material in were developed, among 
the first inventions being those of Swan in England and 
Brush in America. Nearly all of these types of grid 




Fig. 133 —Tudor Plates 
(E.S.B. Co.). 




Fig. 134. — Labora- 
tory Chloride Cell. 



PRIMARY AND STORAGE BATTERIES 



97 




proved inadequate. Some types, however, survived, among 
which may be mentioned the Tudor plate, Fig. 133, and 
the chloride plate, Figs. 

134, 135. With the Tu- «g| ^^BS*jSi :\ ^fr*''- T 
dor plate the grid, after M'lB'flkl 'flfr ■ « J 
being cast, was passed 
through rolls which turned 
over part of each fin, 
giving a grip on the ma- 
terial. With the chloride 
plate the active material 
is made into small blocks 
placed in a mold, and the 
lead grid is cast around it. 
Among the latest devel- 
opments as used by the 
Electric Storage Battery 
Company may be mentioned the box plate. Fig. 136, con- 
sisting of two grids, each having a perforated sheet. of lead 
cast upon one side riveted together 
with the sheet on the- other side. 
This forms a number of inclosed 
pockets which hold the active ma- 
terial. With the continued use of 
the peroxide pasted plate it was 
found that the active material would 
wash off. This led to the use of 
the combination Faure negative 
plate and the Plante positive plate 
for heavy duty, and where the service 
was light, as for automobiles, the 
pasted plate, Exide, Fig. 137, was 
With the Plante type of positive, the peroxide is a 



Fig. 135. — Telephone Chloride Cell. 




Fig. 136. — Box Negative 
(E. S. B. Co.) . 



used. 



98 



EXPERIMENTAL ELECTRICITY 



thin layer, very closely grained, well protected in the inter- 
stices of the plate so that it may not be readily washed 

away. As the peroxide 
gradually disintegrates, 
new peroxide is gradually 
formed by the working of 
the cell upon the grid. 
In America the Man- 
chester type of plate is 
manufactured by the 
Electric Storage Battery 
Company. This plate, 
Fig. 138, consists of a 
grid made of a casting of 
lead antimony that forms 
an alloy more rigid than 
the pure lead and pre- 
vents buckling. The cir- 
cular holes, I inch in di- 
ameter, in this grid are 
Fig. 137. — Exide Battery. '^Wo,^ with spiral buttons 

made of corrugated pure lead ribbon. The but- 
tons are forced into the openings of the 
grid by hydraulic pressure, which se- 
curely locks them in position. During 
the forming process of the Plante plate, 
the buttons expand, resulting in excel- 
lent electrical contact. 

The box negative plates. Fig. 136, of 
the Electric Storage Battery Company 
consist of an alloy grid made with a 
number of small pockets which are filled , , 

^ Fig. 138. — Manchester 

with finely divided porous lead sponge, positive (e. s.b.Co.). 




w» 



PRIMARY AND STORAGE BATTERIES 




Fig. 139. — Rolled 
Negative. 



This lead is not adapted to be mechanically self-support- 
ing, hence it is placed in the pockets of the grid, incased 
in a thin sheet of perforated lead which 
keeps it in position. The box nega- 
tive plate is used with either the Man- 
chester plate or the Tudor positive 
plate, Figs. 136, 138. Tudor positive 
plates are made by the Electric Stor- 
age Battery Company. They are of 
the Plante type, Fig. 133, and consist 
of a single piece of lead with a number 
of vertical ribs extending from face to 
face, allowing thorough circulation of 
the electrolyte between the narrow 
spaces. At proper intervals the ribs 
are supported by horizontal ribs to in- 
sure proper rigidity. 

Rolled negative plates. Fig. 139, con- 
sist of rolled lead having vertical ribs 
on both sides of a center web. The 
ribs are separated by spaces, which in 
the formation are filled with active 
material integral with the body of the 
plate, no material being artificially ap- 
plied. This negative plate is used only with the Tudor 
positive plate. The chloride accumulators. Figs. 134, 135, 
use a Manchester positive plate and a box negative, the 
Tudor accumulator, Fig. 133, contains a Tudor positive 
plate and a rolled negative plate. Another compact form 
of plate, the shelf negative, also made by the Electric 
Storage Battery Company, is shown in Fig. 140. 




Fig. 140. — Shelf 
Negative. 



lOO EXPERIMENTAL ELECTRICITY 



QUESTIONS 

1. What is a primary battery and how is its potential affected by 
kind of electrodes and electrolyte ? 

2. How is the. internal resistance of a cell affected by the size 
of the electrodes, and how would you measure this resistance with an 
ammeter and voltmeter ? 

3. What does the term polarisation mean and how is polarization 
minimized in a single fluid cell and eliminated in a two-fluid cell ? 

4. Draw diagram of cell containing zinc and copper electrodes, 
show positive and negative terminals, direction of current inside of cell 
and in external circuit assuming that current passes from + to — . 

5. How does a storage battery differ from a primary battery? 

6. Explain how lead sulphate enters into the operation of a storage 
cell. 

7. Why cannot the same watt hour output be obtained from a 
cell irrespective of the rate of discharge ? 

8. Draw diagram showing how storage cell is set up to be charged. 
What precautions should be taken? 

9. Give three indications that a battery is charged, all of which 
should be used. 

10. Is the internal resistance of a storage cell high or low ? Would 
the current on short circuit be large or small? 

11. Why in the operation of a large battery is it desirable to 
consider it in an operating way the same as a generator regarding 



CHAPTER VI 

ELECTROLYSIS 

Electrolytic Corrosion. — Too frequently the term elec- 
trolysis is confused with the term electrolytic corrosion. 
While the electrolytic corrosion of water pipes is due to 
electrolysis, caused by stray currents of electricity, the 
term electrolysis refers to any of the processes of disso- 
ciation, such as electric plating, the manufacture of elec- 
trolytic compounds like aluminium, and the electrolytic 
recovery of ores. 

Definition of an Electrolyte. — While an electrolyte, 
though a liquid, is a conductor of electricity, conductors 
are usually solid, have with a few exceptions, such as car- 
bon, positive temperature coefficients, and are not split up 
into component parts during the passage of an electric 
current. Pure copper would be a conductor, whereas 
copper sulphate would be an electrolyte. Some compara- 
tively good insulators when cold become electrolytic con- 
ductors when heated. Various forms of glass containing 
metallic oxides are good insulators when cold, but become 
electrolytic conductors when molten. 

Experiment 54. — Take a small test tube about | inch in diameter 
and fill to a depth of J inch with sodium acetate, powdered. Fig. 141. 
Pass two steel needles through a stopper in the top of the tube, extend- 
ing down to almost the bottom of the tube, so that when the powder 
melts they will make contact, taking care that the needles are not in 
contact. Connect the two steel electrodes to a 116-volt direct current 



I02 



EXPERIMENTAL ELECTRICITY 



service through a i6-candle-power lamp in series with it. Shunt with 



m 



1. 






Fig. 141. — Conductivity 
of Sodium Acetate 
changed with Heat. . 



a piece of wire the two steel needles for an 
instant with power on, noting that the lamp 
lights and that when the shunt is removed 
the lamp will not hght. Heat the tube gently 
with a bunsen burner with the current on the 
circuit. At first the lamp will not light, then 
it will become a dull red, and then it will 
finally light to full intensity. If it is desired 
to repeat the experiment, wash the tube and 
remove the electrolytic deposit from the ends 
of the needles, and be sure that there is no 
moisture in the tube when the sodium acetate 
is added. 

Experiment 55. — Take a fine iron wire. 




Fig. 142. — Positive 
Temperature Co- 
efficient. 



about a No. 30, and coil it up in the form of a helix, mounting it to 

two supports passed through a cork, Fig. 142. 

Support the cork in a clip stand and be sure that 

none of the spirals of the wire touch one another. 

Connect the terminals of the helix in series 

with a galvanometer, shunted, a dry battery, and 

sufficient external resistance so that a full scale 

deflection of galvanometer is obtained. Heat 

the iron wire slowly, taking care not to raise 

it to too high an incandescence lest it burn, and note the decreased 
reading of the galvanometer due to the positive tem- 
per atit?'e coefficient of the wire (lantern experiment) . 

Experiment 56. — Repeat the previous experiment, 
substituting for the iron wire helix two electrodes of 
copper immersed in a solution of copper sulphate. 
Fig. 143. Readjust the resistance in series so that 
now only a small deflection of the galvanometer 

Fig. 143. - Nega- takes place. Heat the glass containing copper sul- 
tive Tempera- phate slowly, and note that the deflection of the 
ture Coefficient, galvanometer increases, due to the negative tem- 
perature coefficient of the copper sulphate (lantern experiment). 



^=^^^ 



Decomposition of Acid Solutions. — Electrolysis is the 
process of separating a compound electrolytically into its 



ELECTROLYSIS 



103 



constituents. Some liquids, notably mercury, will conduct 
electricity without suffering disintegration. When an elec- 
tric current is passed through water slightly acid, the water 
is split up into its two constituent gases — hydrogen two 
parts and oxygen one part, the gases being evolved at the 
electrodes. If these two gases be mixed in the same pro- 
portion and ignited, an explosion will occur and water will 
result. Naturally, the water formed will not occupy the 
same volume as its component gases, but will deposit itself 
in small drops over the surface of the inclosing tube. The 
first evidence of electrolysis was detected in 1800, when 
Nicholson and Carlisle discovered the electrolysis of water. 
Sir Humphry Davy carried on a series of exhaustive ex- 
periments on alkaHne earths, and the caustic alkalies a 
little later, in 1807, producing among other things by elec- 
trolysis sodium and potassium. Faraday, however, was 
the one who first formulated the fundamental law of elec- 
trolysis, 

M = Izt. 



or that the weight of metal deposited in grams M varies 
as the current /, the time / the current is passing, and as 
the electro-chemical equivalent of the substance z. Each 
ampere second, or coulomb, throws out of solution a weight 
of metal equal to its electro-chemical equivalent. Dr. Syl- 
vanus Thomson gives the following values of electro-chem- 
ical equivalents for some of the principal elements : 



Hydrogen . 
Gold . 
Silver . 

Copper (cuprous) 
Tin (stannous) 
Iron (ferrous) 



.000010384 

.0006791 

.001 1 1 81 

.0006562 

.0006116 

.0002902 



I04 EXPERIMENTAL ELECTRICITY^ 

Nickel ..... .0003043 

Zinc 00033698 

Lead 0010716 

Oxygen . . . . . .00008286 

There are two steps which occur in an electrolytic pro- 
cess, the general splitting up of the molecule into its con- 
stituents or ions, and the effect which these ions are likely 
to have upon the electrodes. The first of these steps is 
illustrated in the electrolysis of water, where platinum 
electrodes are used, the nascent g^.^ not attacking the elec- 
trodes. In the following experiment, where copper elec- 
trodes are used, the electrodes are attacked, showing the 
second step in the process. 

Experiment 57. Place two copper electrodes in a projecting tank 
containing a solution of sodium acetate, Fig. 144. The writer prefers 

this to hydrochloric acid, as the acid attacks the sides 

nr T^ of the tank if they happen to be of metal. Insert a 

reversing switch in series with a i6-candle-power lamp 
and a ii6-volt direct current service, Fig. 145. When 
the current is on, notice that gas only rises — oxygen 
— from the positive electrode, the hydrogen evolved 




Fig. 144. — Elec- at the negative electrode attacking the copper elec- 
trolysis Project- trode. Throw the reversing switch, changing the 
ing an . direction of the current through the cell, and notice 

a black deposit rise from the former negative 
electrode. This is some form of hydride of 
copper which is being carried up by the oxy- 
gen gas. This experiment proves the second 
step in the electrolytic process, namely, that 
the ^;//^//j- are going to the anode, or positive ^J^- '45-- Decomposi- 

^ * 1 tion of Acid Solutions, 

electrode, and that the kations as electro- 
positive are being attracted to the kathode or negative electrode. 

The metals and hydrogen are kations and always travel 
to the negative electrode or with the current, while oxygen 
and the chlorides are anions and travel to the positive elec- 



ELECTROLYSIS 



05 



trode. The terms anions and kathions were suggested by 
Faraday. 



0' 



# 



/ 



3l 



TII6 VOLTS 



Experiment 58. Repeat the previous experiment, using a tank with 
a partial partition, Fig. 144, platinum electrodes, the reversing switch, a 
i6-candle-power lamp, and a few drops of 
phenol-phthalein, dissolved in alcohol, to 
the solution. Do not add too much phe- 
nol-phthalein, or the liquid will be too 
cloudy to appear well on the screen. Turn 
on the current and notice that after a 
time bubbles of gas will arise from one 
electrode, the sodium acetate solution at 
th'e other electrode becoming a deep crim- 
son. Throw the reversing switch, shake Fio. 146.- Hoffman Voltameter, 
the tank slightly, and notice that the crimson liquid clears up on one side, 
and that now the liquid on the other side of the tank becomes crimson. 
Experiment 59. To show that oxygen and hydrogen are liberated 
when a dilute solution of sulphuric acid and water is electrolyzed and 
that they are liberated in the proportion of two 
r\ parts of hydrogen and one of oxygen, recourse 

may be made to the Hoffman voltameter, Figs. 
146, 147, consisting of two tubes connected 
through a U to another tube containing a res- 
ervoir at the top. The two main tubes A^ D, Fig. 
146, 147, are graduated similar to a burette and 
have pet cocks at their tops to emit the gases 
when desired. At the bottom of the U another 
pet cock is inserted so that the liquid may be 
drained from the tube. Fill the Hoffman ap- 
paratus with water slightly acid by pouring the 
solution in the reservoir B, turning off the pet 
cock at the bottom of the U and at the top of the tubes. Turn on sep- 
arately and slowly the pet cocks at the top of each tube, allowing the 
acid solution to rise until it fills the tubes, and then turn off the pet cocks. 
Do not have too much extra liquid in the reservoir when the main tubes 
are filled with liquid, or, when gas is developed, there will be no room 
for the Hquid driven out of the tubes, and it will rise over the top of the 
reservoir. In the bottom of each of the main tubes is a platinum elec- 



^ 



J L 116 VOLTS 

^- -^ d.c. 



Fig. 147. — Electro-chemi- 
cal Equivalent of Hy- 
drogen. 



I06 EXPERIMENTAL ELECTRICITY 

trode extending through the glass to terminals. These terminals 
should be fastened to binding posts mounted on the base of the appa- 
ratus, so that connections cannot be made on the fragile inlet wires, but 
must be made at the binding posts. Connect the binding posts of the 
voltameter in series with a i6-candle-power lamp and a ii6-volt direct 
current source of potential. Gases will be developed, hydrogen rising 
at the negative electrode and oxygen at the positive electrode. When 
sufficient gas has collected, read graduations on the tubes, and notice 
that the volume' of hydrogen is twice the volume of oxygen. 

Experiment 60. Test the oxygen by holding over the tube a small 
splinter of wood which has been previously lighted but which has had 
the blaze blown out, leaving only a few sparks remaining. Be sure that 
no liquid, not even a small drop, is left in the top of the test tube, or, 
when the pet cock is turned on, the drop of liquid will be forced by the 
escaping gas against the spark on the splinter of wood, extinguishing it. 
When all these precautions have been taken, turn on the pet cock, hold- 
ing the piece of wood containing the sparks in close proximity to the 
outlet ; the splinter will immediately burst into flame. 

Experiment 61. Test for hydrogen by holding a small test tube one 
half an inch in diameter and three inches long over the opening of the 
hydrogen tube so that the gas will enter near the middle of the tube. 
When the gas has been passing into the tube for a brief time, place 
the thumb quickly over the mouth of the tube, and then bring the mouth 
of the tube near a bunsen flame, at the same time quickly removing the 
finger. A small explosion will result, owing to the mixture of the hydro- 
gen gas with the air. 

Experiment 62. Repeat the experiment of developing the gases with 
an ammeter in the circuit, noting the volume of gas developed in a given 
time and the current consumed. Calculate the ampere seconds, or 
coulombs, and read the volume of gas. When reading the true volume 
of the gas, turn on the pet cock at the bottom of the U-tube and the 
pet cock on the tube containing gas which is not being tested, and allow 
the liquids in the reservoir tube and the untested tube to fall until the 
liquid in the three tubes is on the same level. Read the temperature of 
the gas tested by suspending a thermometer alongside of the tube being 
tested, and read the barometer. Reduce the temperature to zero and 
the barometer reading to 30 inches, calculating the true volume at this 
temperature. Divide the volume of the gas, corrected, into the coulombs 
used, and determine the electro-chemical equivalent of hydrogen. 




ELECTROLYSIS IQ'J 

Small polarity indicators, Figs. 148, 149, depend for 
their operation upon the principle of electrolysis, the 
liquid in the indicator 
turning red at the 
positive electrode 

when the indicator fig. 148. — Polarity indicator (M. E.S. Co.). 

is placed across a ii6-volt direct current circuit. 

Metallic Salt Solution. — With a metallic salt solution the 
process of electrolysis may be termed complete, as the con- 

^ V centration of the solution remains the 

° \J: zJ same at all times, owing to the fact that 

RED 

Pig 149— Polarity ^^ positive clcctrodc is dissolved into so- 
indicator. lution in the electrolyte at the same rate 

that metal is deposited upon the negative electrode from 
the solution. With copper sulphate, for instance, em- 
ploying copper electrodes, metallic copper is deposited 
upon the negative electrode as metallic copper is dis- 
solved from the positive electrode. When the molecule 
of copper sulphate, CuSO^, is split up by electrolysis into 
metallic copper, Cu, and sulphion, SO4, the metallic copper 
is attracted to the negative electrode, where it is deposited 
upon the electrode ; the sulphion is attracted to the positive 
electrode, where the sulphion, having a strong affinity for 
the copper positive electrode, dissolves it into solution. 
The reaction may be written as follows: 

CuSO^ = Cu + SO4. 
SO4 + Cu = CuSO^. 

The electrolytic refining of copper is carried on extensively 
by this method. In such refining are required a large 
number of tanks containing CuSO^, together with the elec- 
trodes of copper, one of which is very thin and of pure 



I08 EXPERIMENTAL ELECTRICITY 

copper, the other very thick and of cast copper ingots. 
From the residue of the positive copper ingots it is possible 
to separate enough gold and silver to cover the entire cost 
of the refining, and the copper obtained is purer than the 
Matthiesen standard. 

Electroplating. — A large industry has arisen during the 
past ten years, depending for its activity upon the electro- 
deposition of metals. Copper, gold, silver, nickel, and 
brass are some of the metals which may be plated by 
electrolytic processes. 

Copper Plating. — Copper plating is used extensively in 
the manufacture of electrotypes for printing. A wax im- 
pression is made from the type when set up, this impres- 
sion being coated with a conducting film of copper. This 
film is obtained by washing the impression with a mixture 
of iron filings and CUSO4. The impression is then placed 
in an electrolytic CuSO^bath and metallic copper deposited. 
When a sufficient thickness has accumulated hot water is 
passed over the thin deposit and it is then easily separated 
from the wax. It is then backed up with metal to give 
rigidity. Articles which are to be copper plated, if non- 
conducting, are treated in a somewhat similar manner, 
namely dusted with a layer of conducting black lead. 
A good copper-plating bath can be made up as follows : 
for each gallon of water 2 ounces of potassium car- 
bonate, 5 ounces of copper carbonate, and 10 ounces 
of cyanide of potassium. About -^^ of the potassium 
cyanide should first be dissolved in a portion of the 
water, and then nearly all of the copper carbonate, which 
has also been dissolved, should be added. The potash 
should next be dissolved and added to the mixture. Cop- 
per or cyanide should be added to the solution after testing 
it, until the deposit is satisfactory. Where copper is 



ELECTROLYSIS IO9 

deposited upon a non-conductor, it is coated with black 
lead. 

Experiment 63. Prepare a standard copper plating solution ; plate 
an electrode, weigh both electrodes before and after, and prove Fara- 
day's law. 

Gold Plating. — Three solutions are in use in gold plating 
to give the different coloring effects known as Cahfornia 
gold, green gold, and red gold. In making the solutions 
the following salts are dissolved in nitro-hydrochloric acid: 
an alloy of 22 parts gold and 2 parts silver is used for 
'California gold ; an alloy of 16 parts gold and 8 parts silver 
is used for green gold; and an alloy of 16 parts gold and 
8 parts of copper for red gold. The chlorides of these 
metals remain upon evaporation, and are dissolved in a 
solution of cyanide of potassium. The anodes must be of 
pure gold. A difference of potential of 5 volts is used in 
the bath. As the rate of solution of the anode is not the 
same as the rate of deposit of the kathode, the solution 
must be occasionally replenished. The electrolyte is not 
circulated as in the case of copper sulphate. 
. Silver Plating. — The standard silver-plating solution 
consists of silver chloride with 9 to 12 ounces of a 98% solu- 
tion of potassium cyanide per gallon of water. Owing to 
a tendency of the silver to deposit in arborescent crystals, 
the articles to be plated have to be kept in motion in a 
plane parallel to the anode surface. The most suitable 
current density for silver plating is i ampere for 60 square 
inches of coated surface. If the solution becomes impov- 
erished or weak, it is necessary to add more cyanide. A 
weak solution is indicated by a violet tinge in the deposit. 
Too much cyanide produces a yellowish or brownish 
color. 



no EXPERIMENTAL ELECTRICITY 

Nickel Plating. — Nickel ammonium sulphate seems to be 
about the best substance to use for nickel plating. Dissolve 
about 1 3 oz. of this salt in a gallon of water, and neutralize 
the solution by the addition of ammonia or sulphuric acid. 
A current density of .4 to .8 amperes for 15 square inches 
of surface and a voltage of 3.5 to 6 volts is a satisfactory 
working formula to use. A good color is given to the 
deposit if the solution is just slightly acid. If it is too 
acid, the deposit will peel; whereas, if the solution is too 
alkaline, the deposit will be dark. 

Brass-plating Solution. — The best brass-plating solution 
contains equal parts of zinc and copper cyanide or car- 
bonate dissolved in ammonium carbonate. A variation of 
these equal quantities will change the color of the deposit. 

Plating E. M. F.'s. — The following voltages have been 
given as most suitable for bath potentials for plating : 



Copper in sulphate 
Copper in cyanide 
Silver in cyanide 
Gold in cyanide 
Nickel in sulphate 



1.5-2.5 volts 

4.-6.0 volts 

I. -2.0 volts 

0.5-3.0 volts 

2.5-5.5 volts 



Given a solution containing a mixture of metals, it is 
possible by using different potentials to separate out each 
by electrolysis. 

Critical Current Density. — The maximum rate at which 
a metal may be taken from a solution and deposited, the 
deposit being of a reguline character, is termed the critical 
current density of that solution. A current value greater 
than the critical value results in depositing the hydrogen 
in conjunction with the metal, forming what might be 
termed a hydride deposit. This deposit is pulverulent and 
will not adhere. The critical current density of a solution 



ELECTROLYSIS 



III 



may be readily determined by means of the simple apparatus 
described in the following experiment. The critical current 
density may be increased by circulating the electrolyte and 
by raising the temperature or increasing the concentration. 



iL 



+ ^ 

16 VOLTS 



Fig. 150. — Critical 
Current Density 
Apparatus. 



Experiment 64. To determine the critical current density of a solu- 
tion an apparatus similar to Fig. 150 may be used. It consists of a 
burette i\ inches in diameter at the top, with a 
pet cock at the bottom which will allow the liquid 
to run out slowly in about 6 minutes. The depth 
of the burette is about 5 inches. For copper 
solutions the central negative electrode consists 
• of a rod of copper \ of an inch in diameter, extend- 
ing down through a stopper into the tube. A 
strip of copper surrounds the rod, thus forming 
the positive electrode. An ammeter and a 32- 
candle-power lamp are connected in series with the 
two electrodes and a ii6-volt direct current source 
of potential. The glass is filled with copper sul- 
phate solution, and the current is turned on, the 
liquid being allowed to run out into the glass. 
The current in amperes is noted when it starts to fall rapidly ; this 
occurs when the liquid is almost out of the tube. Determine the area 
of the dark deposit, not forgetting to include the bottom of the rod, and 
calculate the critical current density in terms of the amperes per square 
inch of surface deposited. Before beginning the deposit be sure that 
negative electrode is clean, and brightly polished. The final deposit 
should be dark at the bottom with a gradual shading up to the reguline 
deposit. If this does not occur, leave the tube full of liquid for a time 
and see with the current on whether a smooth plating of copper over the 
whole rod occurs. If it does not, and the solution is new, see if the rod 
has not been made the positive electrode instead of the negative 
electrode. 

ELECTROLYTIC PRODUCTS 

Alkalies and Bleach. — Caustic soda or sodium hydrate 
is made by the electrolysis of common salt solution. Salt, 
when electrolyzed in the presence of water, forms caustic 



112 EXPERIMENTAL ELECTRICITY 

soda, but during this reaction other compounds in the form 
of a mixture of salt, caustic, and hypochlorate of soda are 
found. In practice the combination is avoided by the use 
of a porous diaphragm, or by drawing off the caustic soda 
solution as soon as formed, or by absorbing the metallic 
sodium by mercury, as in the Casner process using molten 
lead. Upon the passage of a current using a carbon 
positive electrode, chlorine is developed ; this is conducted 
off in gaseous form to chambers containing lime, forming 
bleaching powder. When the mercury combines with the 
metallic sodium as in the Casner process, it may be sepa- 
rated from the mercury by washing. By using an iron 
electrode in a solution of water, caustic soda is developed 
by the secondary reaction. The Casner process is em- 
ployed in this country at Niagara Falls. A voltage of 4.3 
volts is used at the terminals of the cell. A complete 
description of this process may -be found in Foster's 
Handbook, page 1240, as described by Professor F. B. 
Crocker. 

Sodium. — This is manufactured by the electrolysis of 
caustic soda. At Niagara Falls an iron electrode or vessel, 
constituting the kathode, is employed. This vessel con- 
tains the electrolyte in a fused condition in which dip the 
anodes in the form of rods. The sodium after being 
deposited rises to the surface of the liquid, where it is 
skimmed off. After deposition the sodium is reduced to 
sodium dioxide by spreading in trays, which are placed in 
a tube supplied with air and dried over calcium chloride. 
The sodium peroxide is then used in producing peroxide 
of hydrogen by mixing it with sulphuric acid at zero tem- 
perature, owing to the unstable character of the hydrogen. 
The process for the electrolysis of metallic sodium is 
known as the Casner process and requires 4.4 volts. 



ELECTRO L YSIS 1 1 3 

Aluminium. — The Pittsburg Reduction Company at 
Niagara Falls manufactures this substance in large quan- 
tities by the electrolytic reduction of alumina. The double 
fluoride of aluminium and sodium, in a fused state, is 
used as a dissociant. The tanks consist of carbon slabs 
containing the electrolyte, which is maintained in a fused 
state by the passage of an electric current. The carbon 
constitutes the kathode, and carbon rods also constitute the 
anode, dipping in the electrolyte. The metal is drawn off 
daily from taps leading to the bottom of the cavities in 
the tank. 

Potassium Chlorate. — The National Electrolytic Com- 
pany of Niagara Falls manufactures this substance by the 
electrolysis of a solution of potassium chloride. The 
kathode consists of wire gauze covered with cuprous, or 
copper oxide, and the anode is of platinum. The cells 
are made of wood covered with lead. The potassium 
chloride is supplied continuously by a pipe leading to the 
bottom of the cell. A pipe leading from the top of the 
cell carries off a mixture of solutions of chloride, chlorate, 
and hydrogen gas. This solution is led to a refrigerating 
tank where the temperature is lowered, the chlorate crys- 
tallizing out. During electrolysis the temperature of the 
electrolyte is maintained at about 50° C. by the reduction 
of the oxides by the liberated hydrogen. About 4 volts is 
used to electrolyze, of which 1.4 volts is required to 
convert the chloride into chlorate, and the remainder to 
develop heat. About 500 amperes per square foot of anode 
is used for the current density. 

Sponge Lead. — The process of making sponge lead is 
employed by the National Battery Company at Buffalo for 
use in storage batteries. Litharge is placed in contact 
with a sheet-lead cathode and electrolyzed in a solution of 



114 EXPERIMENTAL ELECTRICITY 

dilute sulphuric acid in which is suspended the lead anode. 
Another method of producing sponge lead consists in the 
electrolytic reduction of galena. 

THE ELECTRIC FURNACE AND ITS PRODUCTS 

While the treatment of the electric furnace belongs 
under the head of electrolysis, the phenomena which occur 
in the furnace are due solely to temperature effects and 
not to electro-chemical action; they may therefore be 
properly termed electro-thermal actions. The phenomenon 
likely to occur in the furnace will be one of the following : 
heating without fusing, as in the manufacture of graph- 
ite ; heating and chemical change without fusion, as in the 
manufacture of carborundum ; and heating and chemical 
change, as in the manufacture of calcium carbide. As 
no electrolytic action occurs in any of these transforma- 
tions, an alternating as well as a direct current may be 
used. 

Graphite. — It was discovered by Acheson in the manu- 
facture of carborundum that when the temperature was 
carried beyond 250° C. a large amount of graphite was 
formed around the conducting core. It is a well-known 
fact that electric light carbons used in street lamps often 
have a deposit or coating of graphite over the tips after 
burning for a time. Upon investigation it was found that 
graphite was formed by simply heating pure carbon to a 
high temperature, and that a carbide is first formed which 
is afterwards decomposed by the high temperature. In 
practice a metallic salt is mixed with the carbon before 
heating. Three parts of iron oxide is mixed with 97 parts 
of finely divided carbon, which is molded in various 
shapes before being graphitized. 



ELECTRO L YSIS 1 1 5 

With the Acheson Company the articles to be graphitized 
are placed in a furnace between the electrodes, forming a 
pile two feet wide by about 35 feet long, the spaces between 
being filled in with a ground mixture of carbon and car- 
bonates. Through the electrodes are sent 3000 amperes 
at 200 volts at starting. As the graphitizing proceeds, 
the resistance decreases, carbon having a negative tempera- 
ture coefficient, the voltage falling to 80 volts, and the 
current rising to 9000 amperes. 

Calcium Carbide. — Calcium carbide, while classed as an 
explosive, yields when mixed with water a gas which is 
highly luminous. One pound of carbon when mixed with 
water produces about 5 cubic feet of acetylene gas whose 
illuminating power would be equal to about 70 feet of 
ordinary gas. It gives, however, a sooty flame, although 
one of high intrinsic brightness. Calcium carbide is made 
by treating in an electric furnace a mixture of i ton of 
burnt lime and \ ton of ground coke, which produces i ton 
of carbide. The reaction is CaO + 3 C = CaC2 + CO. The 
process was developed by Wilkson in 189 1. At Niagara 
Falls, C. S. Bradley has in use a special form of rotary 
electrode furnace using 3500 amperes at no volts, pro- 
ducing one ton of carbide in 12 hours. 

Carborundum. — This substance, carbide of silicon, CSi, 
is much harder than emery and is used extensively as an 
abrasive. A mixture of 6 tons of pure sand and i\ tons of 
ground coke, mixed with a small amount of salt and saw- 
dust, i^ tons, to make the mixture porous, is heated to a 
very high temperature, forming beautiful crystals of the 
carbide to the extent of about 4 tons. At Niagara Falls the 
furnace consists of a fire brick hearth, 16 feet long, 5 feet 
wide, set loosely together, and solid brick walls 8 feet high 
at each end. The current is led into the furnace through 



Il6 EXPERIMENTAL ELECTRIC/TV 

iron frames in the middle to carbon electrodes forming the 
core. 

Barium Hydrate. — Barium hydrate is made from crude 
barytes, one part of barytes being mixed with three parts of 
barium sulphate, and then heated in an electric furnace. 
Vapor in the form of SO2 is driven off, leaving barium 
oxide. The oxide on being placed in water becomes 
hydrated and is then allowed to crystallize. It is used 
quite extensively in the recovery of sugar from beets and 
also in the manufacture of pigments. 

Miscellaneous Substances. — Barium cyanide is made 
from barium carbonate mixed with coke, producer gas 
being passed through it while it is heated electrically. 
Phosphorus is made from pulverized calcium phosphate 
mixed with coke and placed in an electric furnace. The 
phosphorus is vaporized and collected under water. Corun- 
dum is made by the Norton Emery Company. It is simply 
purified emery, and is made by heating bauxite in an 
electric furnace. Iron and steel are made abroad in small 
quantities by electrolytic processes. 

The Electrolytic Rectifier. — This device consists of a 
cell containing an electrolyte of potassium phosphate 
slightly acid. One electrode is of lead, the other of alu- 
minium which has been macerated in a solution of caustic 
soda. When this cell is in operation, provided the voltage 
be below 200 volts and the temperature be below 40°, it 
will allow current to pass in only one direction from the 
lead to the aluminium. It is claimed that this action is 
due to the formation of an insulating skin of aluminium 
hydrate which skims over the surface of the aluminium 
anode. An alternating current may be rectified to pulsat- 
ing direct current, one half of the alternating current lobe 
being extinguished. The life of the cell is about 500 



ELECT R OL YSIS 1 1 7 

hours, after which the insulating skin of aluminium hydrate 
breaks down. 

Electrolytic Interrupter. — There are two types of elec- 
trolytic interrupter, both simple to build and both operat- 
ing on the same principle. The Wehnelt interrupter, 
Fig. 151, consists of two electrodes, a lead | — i-f 
sheet as kathode and a platinum wire, adjust- | [0 



\/ 



able, as anode. These electrodes are suspended 
in a dilute solution of sulphuric acid. During 
action the electrolyzed gases form on the plat- 
inum point, insulating the circuit for the instant, ^^'^•/^i- — 

^ ^ Wehnelt 

The high temperature of the electrode quickly inter- 
liberates the gas, making the circuit again con- i"Pter. 
ducting. This process of interruption is very rapid, several 
hundred times a second, and makes an interrupter advan- 
tageous for work with induction coils. The Cauldwell 
interrupter is quite similar, except that for the platinum 
point is substituted a test tube containing an electrode of 
lead, a small opening existing in the bottom of the tube 
over which forms the insulating gas. 

The Pail Forge. — A pail forge consists of a cell contain- 
ing two electrodes, one of which is thoroughly immersed 
in a highly conducting liquid, the other electrode just hav- 
ing its extremity projecting below the surface of the liquid. 
The latter electrode is manipulated by hand when the 
potential of 220 volts direct current is upon the circuit. 
The concentration of energy which occurs under these 
conditions is very great, raising the iron electrode manipu- 
lated by hand to the melting point. 

Experiment 65. Fill the bottom of a glass jar with a layer of sand 
to prevent the molten particles of iron from falling and breaking the 
vessel. Place a lead electrode in the bottom of the vessel, connecting 
it to one terminal, the negative of a 220-volt direct current source. 



0* 



Il8 EXPERIMENTAL ELECTRICITY 

Fill the vessel with a salt solution and, with the current on, project 
an iron rod connected to the positive terminal of the service into 

-—240 VOLTS the top of the liquid. At first, the liquid will 

r ''■''■ begin to sputter, throwing out particles of water. 

y\ As the temperature of liquid rises, it becomes 

more conducting, and the iron rod will gradually 
become red-hot, molten particles of iron drop- 
ping from it. Remove the rod when in this 
^D condition and shake it ; a shower of sparks of 

molten iron will follow. Care must be taken 
Fig. 152. — Pail Forge. ^^^ ^^ hrmg the electrodes into contact by 
mistake, or a short circuit will follow. 



QUESTIONS 

1. What effect takes place when an electric current is sent through 
water by means of two electrodes ? 

2. Why is it that gas only comes from one electrode during elec- 
trolysis when copper electrodes are used in a solution of sodium acetate. 

3. Explain the principle of electrolysis which prevents polarization 
in a two-fluid cell. 

4. Explain the electrolytic corrosion of water pipes. 

5. Differentiate between the various thermal and electrolytic actions 
which occur in the manufacture of electric furnace products. 

6. What will happen if too high a current density is used in the 
plating of copper ? 

7. What is the modern theory of electrolysis. 



CHAPTER VII 
THE THREE-WIRE SYSTEM 

When Edison first attempted to solve the problem of 
underground distribution by using the two-wire system, 
Fig. 153, he soon realized that it was limited 
in its application, owing to the excessive amount 
of copper required to prevent the voltage from 
falling too much in the distribution of power. 



■o 
-o- 
o 
-o 



iitvolrs He therefore introduced what is known as the 

Edison Three-wire System, Fis:. 
Fig. 153.— "^ 

Two-wire 154, which has resulted in saving 





■h ~ 



System, about 62.5 % of the copper re- 
quired by the old two-wire system. This 
value assumes a neutral feeder half as large v-SuTsioVTs 

as the positive and negative feeders. It also *- 232^* 

rf.c. 

saves a large amount of eners^y „ 

^ ^-^ Fig. 154. — Edison 

which was formerly wasted Three-wire Sys- 

in overcoming the resistance ^^^' 
Fig. 155.— Three- °^ ^^^ wircs of the two-wire system. The 
wire System two-wire systcm. Fig. 153, consists of two 
enerator). j^^j^s Supplied by a potential of 116 volts, 
requiring but two wires. The three-wire system consists of 
three wires having a potential difference of about 116 volts 
between each of the outside mains and the neutral main, and 
232 volts between the two outside wires. The potentials 1 16 
and 232 were formerly produced by two generators, Fig. 
155, connected between the two legs of the circuits. At 

119 



I20 



EXPERIMENTAL ELECTRICITY 



-O- 



FiG. 156.— Three- 
wire System 
(Converter) . 



JL_J2. 



Fig, 157, — Prin- 
ciple of Three- 
wire System. 



present, the three-wire system is produced by using a 

rotary converter, which charges a storage battery to a po- 

,25 250 tential of 270 volts, the neutral feeder beins: 

OHMS OHMS ' ^ 

connected in at the middle point, as in Fig. 
156. End cells are used in connection with 
lay out as described earlier in the text. 
Various methods are used to balance the 
system, and these will be discussed later. 
Theory of the Three- wire System. — In 
Fig. 157, two ii6-volt lamps are shown con- 
nected in series with the 232-volt circuit. As 
both of these lamps have the same resistance, 
the potential will distribute itself so that there 
is 1 16 volts difference of potential across each 
lamp. Suppose a second pair of lamps be 

connected across the circuit, as in Fig. 158, 
they will likewise light up to full intensity. 
If the central portion of this circuit midway 
"^ ± ~ between each lamp be connected with an 
X, *^T^~*T^ ■ ammeter in circuit, no deflection of the am- 

FlG. 158. — Prm- ' 

cipie of Three- meter will occur, as the potential across these 
wire System. ^^^^ points is the Same, a current of electricity 
only flowing when there is a difference of 
potential. Continuing the neutral, in Fig. 
158, to the bottom of the figure, it may be 
designated as ±, meaning plus or minus, the 
symbol always used for the neutral feeder. 
If we measure the potential between the fig. 159.— Prin- 
neutral feeder and either the plus or the cipie of Three- 

■^ ^ wire System. 

minus terminal, a potential of 116 volts will 
be noted. When the system is balanced, with the same 
number of lamps on each side, no interchange of cur- 
rent takes place in the neutral feeder, and no current 





THE THREE-WIRE SYSTEM 12 1 

would be indicated, as in Fig. 159; but suppose that one 
of these Hghts was in series with two lamps in multiple 
across a 232-volt circuit, Fig. 156. The result would be a 
redistribution of potential, because the resistances of both 
sides of the system were not equal The equivalent resist- 
ances of the two lamps in multiple would be much less 
than that of the single lamp. The potential across the 
single lamp would therefore be much higher than that 
across the two lamps, and the single lamp would burn very 
brightly, whereas the two lamps in multiple would burn 
^ below normal candle power, being below voltage. An- 
other way of expressing the problem would be to say that 
all of the current passing through the two lamps would 
have to pass through the single lamp. Suppose that 
the equivalent resistance of two i6-candle-power carbon fil- 
ament lamps of approximately 250 ohms placed in par- 
allel to be 125 ohms (this is only approximately correct, 
as the resistance of the lamps when burned below voltage 
would be slightly larger), the distribution of potential would 
be, as shown in Fig. 156, 160 volts across the single lamp 
and 80 volts across the two lamps. 

If an unbalanced system of this kind be connected 
through the neutral feeder A, Fig. 160, to the neutral 
point of a three-wire battery system having a 
constant potential of 120 volts on each side of 
the system, there would be immediately a nor- 
mal redistribution of potential across each leg 
of the system, and all lamps would burn with 
normal illumination. The additional current ^^ipTeTf 
required by the unbalance in the system would Three-wire 
be supplied by the storage battery, the cells ^^^^^' 
on the unbalanced side of the syst-em supplying the addi- 
tional load. The ammeter A will indicate this unbalance. 




122 EXPERIMENTAL ELECTRICITY 

It is obvious, therefore, that in the three-wire system no 
current flows in the neutral feeder when the system is 
balanced, and whenever there is an unbalance the neutral 
feeder carries the difference. If the system were com- 
pletely balanced, it is evident that by supplying 240 volts 
to the outside mains the neutral could be dispensed with ; 
the amount of copper required would be 50 % of that re- 
quired if a two-wire system were to supply the same energy 
at 120 volts. In addition to this, 50% of the energy dis- 
sipated would be saved. This condition in practice, how- 
ever, is not stable, hence it is always necessary to run at 
least a small neutral feeder. In practice there might be 
4 concentric feeders (a concentric feeder is a two-wire 
feeder having both conductors stranded, one inside of the 
other and separated by insulating material, the whole being 
inclosed in an insulated lead sheath) of 1,000,000 cm., carry- 
ing 240 volts potential to a distributing point. It would 
be necessary to run at least one i,ooo,ooo-cm. feeder as a 
neutral to produce the three-wire system. For ordinary 
commercial work it is quite a problem to keep both sides 
of the system balanced, for in some cases, where theaters 
have abandoned isolated plants for the Edison Three-wire 
System, the conditions are difficult to meet and occasion- 
ally they require the installation of extra neutrals. Although 
the load on a three-wire system may be completely balanced, 
it sometimes happens that return feed of some railway com- 
pany through the neutral feeder will cause an artificial 
unbalance — the //?-drop in the neutral feeder causes this 
condition. 

Experiment 66, Make a set-up, as shown in Fig. 161, consisting of a 
three-wire lamp board containing three 16 c.p. lamps on each side con- 
nected up to a three-wire Edison system having an ammeter in the neutral 
feeder. Have a switch connected in series with the ammeter. Turn on 




THE THREE-WIRE SYSTEM 123 

all the lamps, open the neutral switch B^ and notice that all lamps burn at 
normal candle-power, provided they are all new. Turn off one lamp 
on one side of the system and notice that the two other 
lamps on the same side burn brighter than do the three 
lamps on the opposite side. Now turn on the lamp 
previously turned off, and turn off a lamp on the oppo- 
site side, noting that the effect is the same as before, 
except that the luminosity of the lamps has reversed. 
Turn off another lamp on the same side, and notice that 
the effect of unbalance increases. When the greatest 
unbalance occurs, close switch B^ and notice that the y\g. i6r. — Ex- 
lamps light to normal candle power. Project the am- periment on 
, meter on the screen and repeat the experiment in a Three-wire 
slightly different mariner. This projecting ammeter System. 
should have a zero at the middle of the scale. When switch B is 
closed and all lamps are out, turn them all on. Then turn off one 
lamp, noting the deflection of the ammeter, which will be approximately 
.5 ampere. As now the neutral switch is connected, all of the re- 
maining five lamps burn at equal candle power. This .5 ampere is 
being carried by the neutral feeder and represents the amount of unbal- 
ance in the system. Turn off two lamps on the same side, and notice 
that the deflection is now twice as great. Turn off three lamps, and the 
deflection is three times as great, all of the current for the remaining 
three lamps on the other side of the system being supplied through the 
neutral feeder. Turn on one at a time the lamps that were turned off, 
and notice that the pointer of the ammeter now comes back to zero. 
Now turn off one of the lamps on the opposite side, and notice that the 
deflection of the ammeter is reversed, the deflection being .5 ampere in 
the opposite direction. In this way it is possible to demonstrate to 
good advantage that when the system is balanced, no current flows in 
the neutral wire, and also that the amount and direction of the current 
flow in the neutral wire depends upon the extent and location of the 
unbalance in the system. 

The National Board of Fire Underwriters will not 
consent to the installation of series multiple systems of 
illumination, owing to the redistribution of potential which 
occurs when some of the lamps are burned out and the 
possible danger arising from too big a voltage on the 



5S 



2s: 



124 EXPERIMENTAL ELECTRICITY 

remaining lamps. It is undesirable to extend two branch 
circuits from a single cut-out with only a comimon neutral, 
_^^ as in Fig. 162, for in case this neutral fuse 
T V blows out, a series mul- _^_^^ 
tiple system results, -^^^^ 
the two branch circuits 
\\\ being in series with ^^^^ 163"! -~Approved 

+ ±- each other. The Sys- Branch Circuits, 

ne 116 "^ . 

Fig 162 — Old ^^^ ^^ employ in such cases is that shown 
style Branch cir- in Fig. 163, in which a separate neutral 
^^^^^' fuse is provided for each circuit. This is 

the system recommended by the National Board of Fire 

Underwriters. 

QUESTIONS 

1. How does the three-wire system compare with the two-wire 
system ? 

2. Why should the neutral of a three-wire system be fused heavier 
than either of the outside terminals of the circuit? 

3. Assume a three-wire system balanced. What effect will stray 
currents of electricity passing through the neutral feeder have upon the 
potential across both sides of the system? 

4. What would be the effect of connecting a i iS-volt lamp or a 150- 
volt voltmeter to the 240-volt terminals of a three-wire system ? 

5. Why are direct current motors of large capacity operated from 
240 volts instead of 120 volts? 

6. Explain how a switch could be arranged so as to change over a 
three-wire system to a two-wire system. 

7. How is the three-wire system produced? 

8. Explain the operation of a balancer. Why are the field windings 
of each machine placed in parallel with each other's armature circuit? 

9. Why is it necessary in a grounded three-wire system to use care 
in handling either of the 240-volt terminals in the vicinity of water pipes ? 

10. How can a three-wire lamp board be connected so that it can be 
used for a two- wire load ? 




CHAPTER VIII 
ELECTRICAL MEASUREMENTS 

Ammeter- Voltmeter Method of Measuring Resistance. — 

The direct, or ammeter-voltmeter, method of measuring 
resistance is probably more generally used than any other 
"for the measurement of resistance. This method employs 
two instruments, an ammeter and a voltmeter, 
which are connected up to the resistance, the 
ammeter in series and the voltmeter in multiple, 
the resistance being connected across the ter- 
minals of a source of potential, as in Fig. 164. fig. 164.— 
With a series connection the same current ^"I'^^^^T^"^^' 

meter Method 

passes through all of the devices forming the of measuring 
circuit, whereas with a shunt connection the de- 
vices are in multiple, the path of the current being divided. 
The terms multiple and sJmnt are synonymous. 

Experiment 6']. As in Fig. 164, connect an ammeter in series with 
a i6-candIe-power lamp to a ii6-voIt source of direct current potential. 
Measure with a voltmeter the diiference of potential across the terminals 
of the resistance. In this case the potential will be the same as the 
potential of the service, since there is practically no drop in the short 
leads used and in the ammeter. Calculate the resistance from the Ohm''s 
law formula 

E 



R 



R' 



This formula is a modification of Ohm's law, where R — 
the resistance of the lamp, /= the current passing through 

125 



126 EXPERIMENTAL ELECTRICITY 

the circuit, and E — the potential difference. R' is the 
resistance of the voltmeter, which in the Weston type of 
instrument is always given on the cover of the box. It will 
be noticed in this formula that the current which passes 
through the lamp, as well as the sHght current passing 
through the voltmeter, is represented by the ammeter 
reading /. The current which passes through the volt- 
meter must necessarily be subtracted from this current 
value to determine the true current value passing through 
the lamp. The current value passing through the volt- 
meter is obviously equal to the potential difference E across 
the terminals of the voltmeter (the same as that across the 
lamp), divided by the voltmeter resistance R' . If the re- 
sistance of the voltmeter is 15,000 ohms, and the voltage 
of the service is 120 volts, -[|^|-§^o" ^^ ^ vevy small quantity, 
and under ordinary circumstances, as when measuring a 
resistance of a few hundred ohms, may be neglected. The 
formula, neglecting the E/R\ then reduces to the ordinary 
form R = E / 1, I = E / R, E = JR. If, instead of a low 
resistance, a resistance of 2000 ohms were measured in this 
method, it is evident that a large error would be introduced 
by neglecting to consider the current which passed through 
the voltmeter. 

^^ E 
Problem. Given the formula ^, a voltmeter resistance of 

~^> 

15,000 ohms, a source of potential of 120 volts, calculate the error in- 
troduced in neglecting the quantity E / R' \n measuring resistances of 
500, 1000, 1500, 5000, 10,000, 15,000 ohms. 

Measurement of the Resistance of a Voltmeter. — The re- 
sistance of a voltmeter may be readily determined by con- 
necting in series with a voltmeter and a source of constant 
potential an adjustable resistance, Fig. 165. If the resist- 



ELECTRICAL MEASUREMENTS 



127 



ance be adjusted until the voltmeter reading, indicating 
the service voltage, falls one half, the resistance will be 
equal to the resistance of the voltmeter. 

^ 18 VOLTS dS. 

This may be explained either on the prin- 
ciple that, having doubled the resistance of 
the circuit, the current passing through the 
voltmeter and the corresponding deflection 

has been reduced one half, or on the 

basis that the voltmeter indicates the 

potential difference across its own 

resistance, and, having halved the 

voltage by doubling the resistance, 

the deflection reduces to one half. 



Fig. 165.— Meas- 
urement of Re- 
sistance of Volt- 
meter. 




— 3 150 + 

Fig. 166.— Weston 
Voltmeter Cir- 
cuits (station 
instrument). 



Experiment 68. Connect a voltmeter to 
a source of potential and read the voltage in- 
dicated. Connect an adjustable resistance, 
such as a plug resistance box, in series with 
it, and adjust the resistance until the deflec- 
tion falls one half. Calculate the resistance of the voltmeter. 
Problem. — With a r5o-volt voltmeter, having a resistance 
of 2000 ohms, what current will pass through the voltmeter to 
produce full scale deflection? 

If an additional binding post be placed on this voltmeter, 
as in Fig. 166, tapping in beyond the main resistance of 

2000 ohms so as to make it a 
double scale voltmeter, how 
much resistance would have 
to be inserted in series with 
the suspension of the volt- 
meter, Fig. 167, provided the 
resistance of the suspension 
was 60 ohms, so that the volt- 
meter would deflect a full 
scale deflection with 3 volts 
applied to the terminals -j- 
and 3 ? 




Fig. 167. — Weston Voltmeter 
Suspension or Moving Ele- 
ment. 



128 EXPERIMENTAL ELECTRIC/TV 

Note. In changing over such a voltmeter, care should be taken to 
connect the new resistance in the circuit beyond the old resistance, 
near the suspension, and also to use a wire for this new resistance 
which will have a negligible temperature coefficient, such as lala wire. 

Voltmeter Method of measuring Resistances. — Resist- 
ances of lare^e maernitude, from looo to 

116 VOLTS d.C. O ^ ' 

Lf^wwJ 1,000,000 ohms, may be accurately measured 

Fig. 168. — Meas- by placing them in series with a voltmeter of 

uring Resistance kuown resistance, Fig. 168, noting the drop in 

potential on the voltmeter from so doing, and 

calculating the resistance according to a simple proportion. 

Experiment 69. Connect a high resistance in series with a 120-volt 
direct current source of supply. Short-circuit the resistance and note 
the voltmeter reading e. Remove the short circuit, and read the new 
voltage reading <?' ; e — e' represents the voltage across the resistance, 
and e' represents the voltage across the voltmeter. According to the 
principle that in a series circuit the distribution of potential is proportional 
to the resistance of the circuit if the current be continuous, the resistance 
may be calculated. 

e' :e - e' -.-.R-.x. 

Comparison of Resistances with Voltmeter. — An unknown 
resistance may be determined in terms of a known resist- 
ance by connecting both resistances in series with each 
other and a suitable source of potential, and measuring with 
a voltmeter the relative differences of potential across the 
terminals of the two resistances, the value of the resist- 
ances being to each other as their relative differences of 
potential. 

Experiment 70. Connect a 1 6-candle-power lamp in series with a high 
resistance voltmeter, about 15,000 ohms, and a source of potential such 
as a ii6-volt direct current Edison service. Why does the lamp not 
light and why does the needle of the voltmeter fall only a scarcely dis- 
tinguishable amount from its position when no lamp is in the circuit ? 



ELECTRICAL MEASUREMENTS 129 

The reason that two resistances may be measured in the 
simple way, Fig. 169, is to be explained by considering 
their relative potential and current relations. 
The same current / passes through both re- +y''^°'"7~] 
sistances, as they are in series with each other. L^TC^ 
The voltage current relations of one resistance 
will be 1= E/R. For the other resistance fig. leg^Show- 

\ ing Distnbu- 

they will likewise be / = E' /R' . As the cur- tioa of Poten- 
rent values / are the same, the following re- ^^^^* 
lationshold: j = e/R, I = E' /R' , 

E/R = E'/R', or E:E' ::R:R^. 

This method is particularly useful in cases where it is 
impossible conveniently to place an ammeter in series with 
the circuit, as in testing the bonds of railway tracks. The 
drop in potential across the bond is measured with a volt- 
meter, and the drop in potential across a length of track 
such that the readings will be the same is determined. If 
this value is greater than 6 feet of track, the bond is de- 
fective. The bond is a copper conducting strip placed 
across the joints in tracks spanning the fish plate to pre- 
vent the current from passing through the plate and the 
retaining bolts. 

Calibration of an Ammeter. — A convenient standard 
resistance to use in connection with the calibration of am- 
meters is shown in Fig. 170. It consists of i 
ohm of Krupp resistance wire, coiled up in a ^ ^ b 
a helix. The object of using this wire is that | \$^ \ 
it has a very low temperature coefficient. A 
standard resistance, as in Fig. 170, should have 
four terminals, A, B, C, D. A and B are the cur- 
rent, and C and D are the potential, terminals, ^ig. 170.— 

-T-1 ' t A ^ -r, ^ ^ ' Standard 

ine current termmals A and B are placed m Resistance. 



I30 



EXPERIMENTAL ELECTRICITY 



series with the circuit, as in Fig 172, and the terminals C 
and D lead off to the voltmeter or whatever potential de- 
vice may be used. Between the terminals 
C and D is included the standard resist- 
ance. In connecting the terminals C and 
Fig 171— Method of ^ ^^ ^^^^ Standard resistance and in sol- 
connecting Con- dering the connections, the wires should be 
coiled away from each other, as in Fig. 
171, so that the potential points will be definitely lo- 
cated. 

To calibrate an ammeter of low range the ammeter 
should be connected in series with an adjustable resistance 
to regulate the circuit, a standard ohm, and the service. 
A standard voltmeter is connected to the potential termi- 
nals of the standard, and the difference of potential is 
noted. If the standard resistance is i ohm, the ammeter 
and voltmeter readings should coincide for 
/= E/\. If the range of the ammeter is 
from 5 to 75 amperes, a lower resistance 
standard may be used. The standard "IcTd" 

should be immersed in kerosene oil, so fig. 172.— Potential 
that its temperature will remain con- 
stant. 

Experiment 71. Make set-up as in Fig. 172, 
using a low range ammeter, about 5 amperes, and 
the low voltage scale of a voltmeter, 3 or 5 volts. 
Also use a standard resistance and an adjustable 
resistance, which may be a lamp board. A three- 
wire lamp board is convenient for laboratory work, 
one side of the board being used at a time, as in 
Fig. 173, or two sides being used by putting in a 
jumper, as in Fig. 174. Vary the load on the am- 
meter, reading the voltage and current values, and 
plot a calibration curve. 



Lav,/^ 




Taps. 




Fig. 173. — Lamp 
Board. 




Fig. 174. — Lamp 
Board. 



ELECTRICAL MEASUREMENTS 



131 



Calibration of Ammeters, Series Method. — For central 
station work it is customary in checking ammeters simply to 



FEEDER LOAD 



1 



V 



ROTAF 



FLEXIBLE 
LEADS 



I | [ |e— SHUNT— ^ 3 I 



ROTARY 
/lETER 



STANDARD 
SHUNT 



STANDARD 
AMMETER 





Fig. 175. — Calibrating Rotary Ammeter. 

place another ammeter in series with the one being tested, 
as in Figs. 175, 176, and to check the two instruments 



FEEDER LOAD 




Fig. 176. — Calibrating Feeder Ammeter. 

against each other. In the laboratory a suitable adjustable 
resistance must be placed in series with the meters. In 
practice this is not necessary, as by properly making the bus 



132 EXPERIMEIVTAL ELECTRICITY 

connections and inserting a jumper in which is the standard 

ammeter, the feeders can be arranged on various busses 

^ so that only the feeder or the converter 

I I containing the ammeter to be checked 

^"^T^ ©— <A>J will be in series with the test ammeter. 
Fig. 177. — Calibrating Experiment 72. Connect two ammeters in 
Ammeters in Labora- series with an adjustable resistance ani the ser- 
°^' vice, as in Fig. 177, one being the standard am- 

meter. Vary the adjustable resistance, and check the two meters against 
each other. 

Calibration of a Voltmeter. — The simplest way of cali- 
brating a voltmeter is to place it in multiple with a standard 
voltmeter which may be connected to a vari- 
able source of potential as a storage battery, 1 [ 
Fig. 178, or a number of dry batteries con- 
nected in series. The voltmeter may also be 
checked against an ammeter by means of a fig. 178.— Caii- 
standard ohm, in the reverse manner to Ex- brating a voit- 
periment 71, or the voltmeter may be checked 
against a standard cell by means of a potentiometer ; the 
latter is the most accurate method. 

Potentiometer Method of calibrating a Voltmeter. — As 
the potentiometers made by the various manufacturers here 
and abroad differ in details, no attempt will be made to 
describe the individual circuits, which may be readily 
understood if the general principle of operation of all po- 
tentiometers is understood. In Fig. 179 is shown a poten- 
tiometer, and in Fig. 180 the circuits of the same as made 
by the Leeds and Northrup Company. 

An elementary diagram of a potentiometer is shown in 
Fig. 181. The apparatus necessary to make the calibration 
consists of a standard cell, a galvanometer, a variable resist- 
ance such as a resistance box with suitable side contact 




ELECTRICAL MEASUREMENTS 



133 




Fig. 180. — Circuits of Leeds and Xorthrup Company Potentiometer. 



134 EXPERIMENTAL ELECTRICITY 

switches, a constant source of potential such as a storage 

battery, and a voltmeter. For a beginner it is desirable to 

^ use a storage battery or a number of dry 

L uTm batteries connected in series as a source 



Ro -^ of potential, although an experienced man 
p>A^Wwv«^^^P may use a direct current lighting circuit 
HJ-<£>--?N ^\y^^ jg slightly variable and obtain the 

Fig. i8i. — Diagram of Same results. 

Potentiometer Cir- An example of the operation of a po- 
tentiometer may be given as follows. For 
the standard cell circuit a suitable resistance is chosen, 
such as -looo ohms. A value of 5000 to 10,000 ohms is 
preferable, but this value is selected to simplify calcula- 
tions. Suppose, after consulting the temperature of the 
standard cell and applying the correcting formula, it is 
found that the potential of the cell is 1.4234 volts at room 
temperature. A resistance of 1423.4 ohms is inserted be- 
tween the contacts B and C on the poten- 
tiometer. Fig. 182, causing a distribution 
of potential of i volt per 1000 ohms over ' ^wIawv 
the resistance included between these two I 

I*- 1423.4 

points when the standard cell connected ^ *^ 

• -1 i.!. 1 . • Fig. 182. — Standard 

m series with the sralvanometer is con- „ „ „. . 

^ Cell Circuit. 

nected between these two points. Sup- 
pose that it is desired to calibrate the voltmeter at 40 volts 
on its scale. This would call for a resistance of 40,000 
ohms in the battery circuit to which the voltmeter is at- 
tached, in order to obtain the same distribution of poten- 
tial, I volt per 1000 ohms, as in the standard cell cir- 
cuit, both sources of potential being connected to different 
parts of the same resistance, as in Fig. 181. In some po- 
tentiometers, as that of Hartman and Braun of Germany, 
resistances of 9 x 100 ohms and 9 x 1000 ohms are in- 



.4234 VOLTS-» 
k-l VOLT— > 



ELECTRICAL MEASUREMENTS 135 

serted at the points B and C, Fig. 182, suitable contact 
switches moving over them so as to facihtate the adjust- 
ment of the resistance in the standard cell circuit. In this 
case, in setting for 1423.4 ohms, one contact is slid over to 
the 1000 mark, the other contact is slid over to the 400 mark, 
and resistance plugs of 23.4 ohms are taken out between con- 
tacts B and C. Care must be taken in the use of this box 
to remember that if a total of 40,000 ohms is to be included 
in the external circuit, there is already in the circuit 
9 X 100 and 9 X 1000 or 9900 ohms, irrespective of where 
contacts B and C may be placed ; and when the setting is 
made as in the above example, there is a total of 9924.4 
ohms in the external circuit. 

For a setting of 40 volts it would require an additional 
resistance of 40,000 — 9924.4 = 30,075.6 ohms. This resist- 
ance is inserted in the adjustable resistance R, outside of the 
standard cell circuit. Fig. 181, When these adjustments have 
been made, the contact switch iVis closed, placing 100,000 
ohms in series with the galvanometer circuit. If the volt- 
meter reading is correct at 40 volts, no deflection of the 
galvanometer will occur, as the difference of potential 
across the terminals of the external resistance 1423.4 ohms 
will be 1.4234 volts, equal to the voltage of the standard 
cell which it is opposing. Suppose, however, that the 
voltage of the circuit was greater or less than 40 volts, 
although the voltmeter indicated 40 volts, then the differ- 
ence of potential on the external circuit would not equal 
that of the standard cell, and a deflection of the galvanom- 
eter would occur. In this case the resistance in R outside 
of the standard cell circuit is adjusted until no deflec- 
tion of the galvanometer occurs on closing switch N. (Do 
not leave the switch ^Y closed longer than is necessary, in 
order to limit the discharge of the standard cell.) When 



136 EXPERIMENTAL ELECTRICITY 

the previous adjustment has been made, close the switch N 
on last contact, eliminating the series resistance of 100,000 
ohms. A finer adjustment of the resistance R in the 
external circuit may now be made until zero deflection 
occurs. If, during this performance, the resistance of the 
standard cell circuit has not been disturbed, the true volt- 
age of the circuit may be obtained by dividing the total 
resistance of the external circuit — not forgetting the 9900 
ohms which is always in the circuit with the Hartman and 
Braun potentiometer — by 1000 ohms, the unit of distri- 
bution of potential. In the adjustment of the box, if it has 
been necessary in order to balance to disturb the standard 
cell circuit, the following relation holds : 

e'.e'::R\R^R\ 
where e = voltage of standard cell, e' = voltage of external 
circuit, R = resistance of standard cell circuit, and R' = ex- 
ternal resistance. 

Experiment 73. Calibrate a voltmeter by the above method. 

Calibration of an Indicating Wattmeter. — As described 
on page 42 a wattmeter has two coils, a series coil and 
a shunt coil. These two coils are connected in the circuit 
in the same manner as an ammeter and a voltmeter are 
connected, the current coil being in series 
and the potential coil being in multiple, as ~?^fe=:A- 
in Fig. 183, where a wattmeter is shown _ ^ 



connected up so as to measure the energy fig. 183.— indicating 
consumed by a 16-candle-power lamp, Wattmeter Cir- 
about 50 watts. Wattmeters are usually 
constructed so as to carry a fixed maximum current through 
their current coils. A 300-watt wattmeter may have a 
current- carrying capacity of 3 amperes, although for short 
intervals it may carry as high as 6 amperes without over- 




ELECTRICAL MEASUREMENTS 137 

heating. A standard ammeter may be placed in series with 
the current coil, and a standard voltmeter may be shunted 
across the potential coil, the product of both instruments 
indicating the true watts against which the reading of 
the indicating wattmeter may be checked. Fig. 184. In 
order to vary the reading of the wattmeter, it is necessary 
to vary the amperes or the volts. A good 
method is to maintain the current constant, 
and then to vary the potential, as in Fie:. 

1 • , . • -, , , Fig. 184. — Calibra- 

185. Here the potential is varied by hav- tion of indicating 
ing an adjustable resistance in series with Wattmeter. 
a lamp, shunting the lamp to the potential coil and the 
standard voltmeter. Varying the resistance in series with 
the lamp will vary the potential. 

Experiment 74. Calibrate a wattmeter, using an ammeter and a 
vohmeter, and vary the potential, Fig. 185. Ob- 
tain several readings over the whole scale of the 

instrument. Change over 

the instrument terminals "*" UjT^ 
Fig. 185. — Calibration and take the average of - 1 




of Indicating Watt- two values. Fig. 186. - Calibration 

'^^^^^■- Experiment 75. Call- of Indicating Watt- 

brate a wattmeter by checking against another meter, 
wattmeter, as in Fig. 186, varying the amperes with an adjustable resist- 
ance such as a lamp board. 

Wheatstone Bridge Method of Measuring Resistance. — 

Given two resistances of 10 and 100 ohms 
connected in series to a dry battery of 2 
volts as in Fig.^187, the 2 volts will distrib- 
ute itself over these two resistances accord- 
^ 10 100 inor to Ohm's law in the ratio of ^V of the 

ly- 10/ O 11 

r T^ • '\ potential across the 10 ohms and \% of the 

Fig. 187. — Prmciple ^ ^ 11 

of Wheatstone potential across the 100 ohms. Suppose 
^"'^se. that the previous combination be shunted by 




10 luu 



138 EXPERIMENTAL ELECTRICITY 

two additional resistances in series of 20 ohms and 200 ohms, 
Fig. 188, then the potential 2 volts will likewise distribute 
,2 VOLTS itself over these two resistances in the 

"" ' ratio of y\ of the voltage across the 20 

ohms, and W of the potential across the 
200 ohms. The distribution of potential 
in a series direct current circuit is al- 
L^^^^^^^^,^,^^.^,,,^^^^^^ ways proportional to the resistance of the 
^ ^y ^°°5/^ parts of the circuit. In the above case 

Fig. 188. — Principle of there will be the same dijfereitce of po- 
Wheatstone Bridge. teiitial across the resistance AB as across 
the resistance DE, namely Jy of 2 volts. If a galvanom- 
eter is connected through a switch between these two 
resistances, as in Fig. 189, and a switch in the galvanometer 
circuit is closed, no deflection of the galvanometer will occur, 
as there would be no potential difference 



1Q B 100 




across the galvanometer terminals, caus- 
ing no current flow. Suppose, however, 
that the resistance DE, Fig. 189, was 100 
ohms instead of 20 ohms, the difference 
of potential across DE would be greater 
than that across AB, and a deflection of fig. 189. — Principle of 
the galvanometer would result, owing to wheatstone Bridge. 
a difference of potential between the points B and E. If 
the resistance DE was changed from 100 ohms to 10 ohms, 
the deflection of the galvanometer would be in the opposite 
direction. When the resistance DE is variable, it can be 
adjusted until no deflection of the galvanometer occurs, in 
which case the ratios of potential would be AB : BC: : DE : 
EF. If AB was 10 ohms, BC was 100 ohms, DE was 25 
ohms, EF, or;r, would have to be 250 ohms. A Wheatstone s 
Bridge consists of this combination of 4 resistances con- 
nected in closed series formation, having a battery con- 



ELECTRICAL MEASUREMENTS 



139 



M t-, N 



t 




nected to two opposite junctions, and a galvanometer 
connected to the two remaining opposite junctions. The 
arms AB, BC are termed the ratio arms. The ratio arms 
and an adjustable resistance, Fig. 190, are 
usually made up in the form of a box termed 
a Post-office Box, or a bridge-testing set. The 
ratio arms, Fig. 191, of a post-office box con- 
sist of two sets of resistances of 10, 100, 

1000 ohms respectively, as arms A and B in ^ 

the figure. By withdrawing plugs in both fig. 190. -Principle 
sides of this box, the ratio of both sides of wheatstone 
may be readily changed. The box contains " ^^' 
an adjustable resistance C, whose resistance may be made 
anything from .1 ohm up to 20,000 or more ohms by suit- 
ably manipulating the plugs. One extremity of this adjust- 
able resistance may be con- 
nected to one end of the ratio 
arm B, and the other end of the 
adjustable resistance C may be 
connected to one terminal of 
the unknown x, the other ter- 
minal of the unknown being 
connected to one extremity of 
the ratio arm A. This gives 
four resistances in series. Two 
switches are provided on the instrument so that the battery 
can be connected to two junctions, skipping a junction 
through a switch, and the galvanometer can be connected 
to the two remaining junctions, skipping a junction, through 
a switch. To operate the box a suitable ratio is taken out 
in the ratio arms, such as 10 and 100 ohms. Proper con- 
nections are made between the unknown x, the resistance, 
the battery, the galvanometer, and the jumper connection. 




Fig. 191, — Post-office Box. 



140 EXPERIMENTAL ELECTRICITY 

K guess resistance of about 200 ohms is taken out in the 

adjustable resistance C, and the battery and galvanometer 

switches are closed for an instant. If the experimenter is 

unfamiliar with the method, it is well to shunt 

the galvanometer with a shunt of about 20 ohms, 

20 OHMS ^g -j^ Yiv. 102. The deflection of the galva- 

FlG. 192. . . 

Galvanometer nometcr is notcd, and a readjustment of the plugs 
Shunt. is made, gradually reducing the magnitude of the 
deflection. When the deflection moves in the opposite 
direction, it will be known that too much resistance has 
been changed. The resistance of the unknown is cal- 
culated from the previous data, and the ratio arms may 
then be changed if desired so as to give greater refine- 
ment still, using a ratio of 10 : 1000 instead of 10 : 100. In 
setting up a resistance to measure by the Wheatstone 
bridge method, it is not necessary to have a special resist- 
ance box, as any four resistances connected up in a closed 
series formation, as in Fie;. 193, will serve the a b 

purpose, provided the resistances have the I J 

proper range. It is well to bear in mind that 
the battery should be connected on two junc- 
tions not adjoining and the galvanometer on 
the two remaining junctions. It is also well P^^'^^"^'''"^ 
in making the preliminary connections to have fig. 193. 

some resistance in each of the arms so as not to Various Arrange- 
ments of Four- 
short-circuit the battery or cause excessive de- series Resist- 

flection of the galvanometer with accidental ^nces. 

closing of the switches. When through with the resistance 

box, always return the plugs to their proper positions, giving 

them a slight twist with pressure to insure good contact. 

This will keep the box in good condition. 

Various modifications of the Wheatstone bridge are in 

use, known as the sUde wire bridge, the roller bridge, the 






Hh 



ELECTRICAL MEASUREMENTS 141 

Carey-Foster Bridge, and the Thomson Double Bridge. 
The Wheatstone bridge is accurate for measuring resist- 
ances from about 2 ohms to 200,000 ohms. Below 2 ohms 
to .1 of an ohm, the direct method may be used, substitut- 
ing a galvanometer for a voltmeter. From .1 of an ohm 
down, the Thomson Double Bridge may be used. It is 
possible with this bridge to obtain an accuracy of .01 ^ in 
measuring resistances as low as .05 ohms. Directions for 
using the Thomson Double Bridge are given later. 

Slide Wire Bridge. — The slide wire bridge differs from 
the ordinary form of Wheatstone bridge in that a fixed 
resistance wire is substituted for the ratio arms. A sliding 
contact connected to one terminal of the galvanometer 
moves over this wire. For the adjustable resistance is sub- 
stituted a fixed resistance O, as in Fig. 194, 
the unknown resistance being inserted in the 
same way as in the ordinary bridge. The 
battery connection is made across the two ex- 
tremities of the stretched wire, and the other 
galvanometer terminal is connected midway ^^^- 194- — Slide 

, 11 11^-,. Wire Bridge. 

between the unknown and the fixed resistance. 
To measure the unknown, the sliding contact is moved over 
the stretched wire until a balance occurs. A scale accom- 
panies the stretched wire so that, when a balance occurs, 
the ratio of the two parts, A and B, may be noted. The 
ratio of these two parts will be the same as the ratio of 
their relative resistances, the wire being of uniform resist- 
ance. The value of the unknown is then found from the 
simple proportion : 

A\B\\0\x where x is the unknown. 

Experiment 76. Stretch a German silver wire between two contact 
points, as in Fig. 194, and insert a known and an unknown resistance, 
such as a i6-candle-power lamp. Calculate resistance of lamp. 




142 



EXPERIMENTAL ELECTRICITY 



The Wire Roller Bridge. — This form of bridge is similar 
to the sHde wire bridge, except that the sHde wire is coiled 
upon a drum, as in Fig. 195 a, both extremities of the wire 
being fastened to metal pegs which are 
electrically connected to a brass axle pass- 
ing through the drum. This axle is divided 
into two parts at its center, so as to sep- 
arate, electrically, the ends of the wire. 
Pressing upon the axle are two upright 



s 



-n 



fQ 




100 X 
Fig. 195 b. — Wire 
Roller Bridge. 



Fig. 195 a. — Wire springs, which continue the electrical con- 
nections to the two binding posts, P and R. 
The stretched wire, which may be placed in the circuit by 
connecting to the binding posts P and R, constitutes the 
ratio arms, Fig. 195 b. A small contact 
wheel, electrically connected to the binding 
post N, moves over the slide wire, shding 
along an axle supported in spring supports. 
Between the binding posts P and R are in- 
serted a standard resistance and the un- 
known resistance, the battery connections 
and galvanometer connections being as shown in the figure. 
Sometimes the standard resistance, consisting of three re- 
sistances, 10, 100, and 1000 ohms, is mounted 
in the base of the roller bridge. With this 
method. Fig. 196, it is only necessary to in- 
sert the unknown between the two binding 
Fig. 196. — Wire posts provided for it. It is sometimes con- 
Roiier Bridge. yenient to substitute for the battery the sec- 
ondary terminals of an induction coil, Fig. 197, the induc- 
tion coil being operated from a vibrator and a dry battery, 
and to substitute for the galvanometer a telephone receiver. 
As the drum is turned, the alternating difference of poten- 
tial of the bridge, due to unbalance, causes a sHght tone in 





ELECTRICAL MEASUREMENTS 143 

the receiver. With the continued turning of the drum, this 
tone gradually decreases as the point of balance is reached. 
When the balance point is passed, the tone in- 
creases again. The balance is set at the point 
of minimum sound. This method or modifica- 
tion of the roller bridge is particularly advan- 
tageous in measuring the resistances of elec- 
trolytes as in Fig. 197, as it eliminates the 
polarization e. m. f. yiq. 197. — Re- 

Experiment 77. Compare with a roller bridge con- t!^'*^!^^^, ^ ° 
nected up with a telephone and an induction coil, as in ' ' ' 

Fig. 197, the relative resistances of copper sulphate solution and salt 
solution, taking the salt solution as a standard. 

Carey-Foster Bridge. — This bridge is used for the com- 
parison of standards. The bridge, Fig. 198, consists of 

ll four arms. A, B, C, and X. A and B are usu- 

wvTpvvw ually equal, of 10 ohms each in some bridges ; 

^1 ZJ C and Xto be compared should be almost equal, 

N C being another standard resistance. To one 

^ '^ \ extremity of C and one extremity of X is con- 

Carey-t oster J -J 

Bridge. ncctcd a strctchcd standard wire, whose resist- 
ance per unit length is accurately known. A movable 
contact N is slid over this wire when an adjustment is 
being made. Two commutating switches are provided, 
one for commutating or reversing the battery connections 
to eliminate thermal effects, the other to substitute the 
resistance C for the resistance X. Four observations are 
taken on the galvanometer, connected in the regular bridge 
manner, balancing each time. Two of these readings are 
taken with C and X, as in the diagram, the battery being 
commutated after the first reading. The average of these 
two values on the vernier at N gives setting 5. The 
standards are then commutated, X being substituted for C, 



144 EXPERIMENTAL ELECTRICITY 

and two more readings taken, the battery being commu- 
tated after the first reading. This will give an average 
reading of the two vernier settings of 5'. The difference 
between 5 and 5' represents a certain length of the slide 
wire which is equal, at that particular temperature, to the 
difference in the resistances of the two standards. If <^ is 
the coefficient of resistance of the slide wire, the result 
may be obtained from the following expression : 

X= C-\-<l^{S' - S). 

In using this formula care, must be taken to see that the 
standard and Jf have the correct relation to each other. 
If they changed places in the set-up, the formula would 
become 

X=C- cl^iS' - S). 

Experiment 78. Compare a standard ohm with a similar resistance, 
taking care to correct the standards for temperature. 

Thomson Double Bridge. — The Thomson double bridge 
is a modification of the Wheatstone bridge, having four 
resistance arms connected up in a manner similar to the 
ordinary bridge. Between the unknown resistance and 
the adjustable resistance is placed a heavy contact to reduce 
the resistance of the joint to a minimum. This connection 
is shunted by two other resistances having the same ratio 
as the ratio arms. The galvanometer connects midway 
between these two resistances and the two ratio arms. If 
A and B are the ratio arms, if C and D are these two 
auxiliary resistances, and E is the value of the adjustable 
resistance when a balance is made, the following expression 
should hold, 

A:B::C:E>::E:X. 

Insulation Test. — When large quantities of electric light 
wire are used by the operating companies, it becomes nee- 



ELECTRICAL MEASUREMENTS 145 

essary to test the insulation at frequent intervals to discover 
defects in manufacture. A simple method of doing this is 
to place a sensitive galvanometer in series with the insula- 
tion and a source of potential, noting the deflection of the 
galvanometer. Knowing the constant of this instrument, 
the resistance may be readily calculated. 

Experiment 79. Coil up about 500 ft. of No. 16 insulated copper 
electric light wire, bring out the two free ends, and solder them together. 
Place this coil in a vessel containing a salt solution and 
introduce a metal electrode into the bottom of the vessel. 1^ 

Place the two electrodes in series with a 240-volt direct 
current source of potential, and a galvanometer, taking 
care to short-circuit the galvanometer so as to prevent an 
excessive kick of the galvanometer owing to the charging 
capacity effect of the cable. After a time, open the short- 



(^ 



circuiting switch, and note deflection of the galvanometer. Fig. 199. 
If the galvanometer is of the ballistic type, this deflection Insulation 
will be quite small. The resistance of the insulation should ^^ ' 

be quite high, 240 megohms per mile at least. A megohm is a million 
ohms. Having calculated the resistance of the insulation by noting the 
deflection, applying the constant of the galvanometer to determine the 
equivalent current value, and applying Ohm's law, calculate the resistance 
of one mile of wire. In so doing, do not forget that insulation is in 
parallel aind therefore that the resistance per mile will be less than the 
resistance of 500 feet. 

The Galvanometer. — The galvanometer is one of the 
most sensitive instruments in use for measuring current. 
In construction it is quite similar to a Weston voltmeter, 
except that a flexible suspension is substituted for the 
spiral springs, and the resistance of the instrument is low, 
varying from 60 to 100 ohms. Referring to Fig. 200, it 
will be noted that the instrument consists of steel magnets 
surrounding a movable suspension. Inside of some sus- 
pensions is placed a soft iron core, which tends to facilitate 
the passage of lines of force. In the ballistic type of 



146 



EXPERIMENTAL ELECTRICITY 



instrument this core is eliminated. A telescope is used 
in connection with the galvanometer, a set of cross hairs 
being placed across one of the lenses of the telescope. 




Fig. 200. — Galvanometer. (O. T. Louis Co., N. Y.) 

Upon the telescope is also mounted a scale which is 
reflected from a small mirror placed on the galvanometer 
movement. With a deflection of the galvanometer, this 
scale seems to move across the field in the eyepiece, the 
reflection traveling. 

How to set up Galvanometer. — To a beginner who is 
unfamiliar with a galvanometer, its setting up affords some 
difficulty, especially as the fiber of the galvanometer is so 
likely to fracture. The galvanometer should first be taken 
from its case and mounted upon a table about two feet 
from the edge. The set screws should be adjusted until 
the instrument is perfectly level. The suspension should 
then be released by the releasing device on the instrument. 
When this is done, it will probably be noticed that the gal- 
vanometer movement will not be entirely free, but will 
touch upon one side. Carefully adjust the set screws until 



ELECTRICAL MEASUREMENTS 147 

the movement is free, taking care not to touch the fiber or 
the movement with the hands in any way, as the fiber is 
very easily broken, and very hard to repair. When the 
suspension is free and moves freely, point the telescope in 
the direction of the mirror, placing the eye on a line with 
the barrel of the telescope, first in a vertical, and then in 
a horizontal direction, scanning alongside — not through — 
the telescope. When this adjustment is made, look 
through the telescope and adjust it until the scale comes 
into focus. It may be that, after all of these precautions, 
only the mirror and not the scale may be seen through the 
telescope. This is due to the fact that the mirror is turned 
too far. In this case the suspension of the instrument 
must be slowly turned until the scale comes into view. A 
very small movement of the fiber will cause a very large 
movement of the mirror. After having first set it, do not 
shift the telescope in order to find the scale, or the finding 
will be impossible. 

Determination of Resistance of Galvanometer. — The re- 
sistance of an ordinary galvanometer is about 100 ohms. 
A simple way of measuring the resistance of a galvanome- 
ter is to place in series with it a very high resistance, such 
as a graphite stick containing 500,000 ohms, and a dry 
battery. Note the deflection. Then shunt the galvanome- 
ter with a resistance equal to the galvanometer, in which 
case the deflection will fall one half. Graphite sticks are 
very convenient things to have in the laboratory. They 
can be purchased from the Dixon Crucible ^^^^^^ 
Company of New York for about 18 cents, rx^rr^^^^"^ 
They come in various sizes, anywhere from ' — ^ 

. , , r . 1 • , . 1. FIG.201.— Meas- 

an eighth of an mch to an inch m diameter ; ^-ing Resist- 
of any resistance from a few hundred ohms ance of Gai- 

tO 700,000 ohms. The cost of the rods does vanometer. 



148 EXPERIMENTAL ELECTRICITY 

not depend upon the resistance of the stick, but upon its 
dimensions. The set-up for the above experiment is shown 
in Fig. 201. 

Determination of the Constant of a Galvanometer. — The 
constant of a galvanometer is expressed in two ways ; it 
may mean the amperage necessary to cause one scale de- 
flection, or it may be, as in the case of a ballistic galva- 
nometer, the microvolts necessary to produce one scale 
deflection. This value is usually expressed by the symbol 
K. Where K means the amperes per scale deflection, it 
may be readily determined by placing a low potential, such 
as i^^oo" ^^ ^ ^^o\\., across the terminals of the galvanometer, 
noting deflection. The resistance should then be calculated 
according to the previous experiment. The potential e di- 
vided by the resistance of the galvanometer r will give the 
current passing through the galvanometer. Dividing this 
quantity again by the scale deflection 6 will give the am- 
peres K per scale deflection : 

K = 



rxe 

Experiment 80. Connect across the terminals of a dry battery 1000 
ohms and from i of these ohms lead off to a galvanometer, as in 

I Fig. 202. Usually a resistance box is used, Fig. 203, and 
the 1000 aiid i ohm plugs are pulled out, placing looi 
ohms in the circuit. This introduces a small error which 

--^AA^AA/^ I is practically negligible. A telephone 

plug with a small binding post on its top 

may then be inserted in the 2-ohm place 

in the resistance box, which usually ad- 

"^^ joins the i-ohm position. This will allow 

, , ■ ^ ,^' wires to be conveniently connected both 
Method of . , . , , ^ -r^- ^ 'ooo 

r»Ktoir.^r,rr sides of the I ohm. See Fioj. 203. De- ^^.^ t 

Obtaining ^ & J FiG. 203. — Low 

Low Po- termine constant A' of any galvanometer Potential Ob- 
tential. which may be convenient, using set-up as tained. 





ELECTRICAL MEASUREMENTS 149 

indicated in Fig. 203. If galvanometer is very sensitive, it may be 
necessary to insert a greater resistance in the battery circuit than 1000 
ohms in order that the galvanometer deflection will not be too great, 
making proper correction in formula. 

Measurement of Capacity. — The most satisfactory way 
of measuring capacity with a direct current circuit is by 
means of the direct discharge method. The condenser 
under test is charged from a battery of known potential 
and then discharged through a ballistic gal- 
vanometer as in Fig. 204. When the switch 
B is up, Fig. 204, it makes contact with A 
and charges the condenser. When moved 
down, it opens the battery circuit and dis- fig. 204. — Meas- 
charges the condenser through the galva- urementofCa- 
nometer. The kick of the galvanometer ^" 

should be read. Do not wait for a steady deflection. For 
the unknown condenser a standard condenser is then sub- 
stituted with the same set-up, including battery, and experi- 
ment repeated. The relation between the two condensers 
will be the same as their relative deflections : 

where C and C are the capacities in farads or microfarads 
and and Q^ are the relative deflections. 

Experiment 81. — Compare two condensers by the direct discharge 

method, using set-up similar to Fig. 204. If unknown condenser yields 

too large a deflection to be read on the scale of the galva- 

^J^_<3_^ nometer, the charging potential may be reduced by shunting 

^t^^J^^_Qjp off" from a part of a resistance connected across the terminals 

Fig. 205. of a battery. A double throw- switch. Fig. 205, may be used 

Substituting in this experiment to rapidly substitute one condenser for 

S%vitch. another. 

Commercial Testing Sets. — Many forms of Wheatstone 
bridge are made up in compact style for commercial test- 



ISO 



EXPERIMENTAL ELECTRICITY 




Fig. 206. — Queen Slide Wire Bridge. Fig. zq-j. — Queen Acme Testing Set. 




— Queen Decade Testing Set 




Fig. 210. — Queen Wlieatstone Bridge 



Queen Laboratory Wheat- 
stone Bridge. 

ing. These may be of the 
slide wire type, Fig. 206, em- 
ploying a telephone receiver 
which clicks when the slide 
wire circuit is completed, or 
the galvanometer may be con- 
structed as part of the instru- 
ment, as Figs. 207, 208 ; or the 
bridge may be in a simple 
compact laboratory form 
as Fig. 209 ; or the resist- 
ances may be multiplied on 
the decade principles in 
multiples of ten, as Pig. 210. 



ELECTRICAL MEASUREMENTS 151 

All of these forms of bridge may be obtained from O. T. 
Louis Company, 59 Fifth Ave., New York City, agents for 
Queen and Company. 

QUESTIONS 

1. Describe the potentiometer method of calibrating a voltmeter. 

2. Explain in detail the practical meaning of Ohm's law as applied 
to all bridge methods. 

3. Draw a diagram of circuits for testing an ammeter. 

4. How would you calibrate an indicating wattmeter? 

5. Why is it desirable to use a short-circuiting galvanometer switch 
in testing a cable ? 

6. How would you set up a galvanometer and determine its con- 
stant? 

7. Why is the Thomson double bridge so accurate for measuring 
small resistances ? 

8. What do we mean by the current-carrying capacity of wires, and 
why should we never send more than .2 ampere through a low resist- 
ance in a post-office box? 

9. Take resistance from 500,000,000 ohms down to .005 ohm in 
suitable steps and give the proper measurement method to use in each 
case. 

10. Why should you always have a few plugs out in the adjustable 
resistance of a bridge when you are connecting the galvanometer to the 
bridge circuit? Why is this also desirable in the ratio arms or battery 
circuit, especially if only one contact key is used? 



CHAPTER IX 
THE SHUNT MOTOR 

Given a motor such as in Fig. 211 (a shunt CQ motor 
installed on the ceiling, G. E. Co.), the question arises as 
to how to determine whether it is a shunt motor ; if so, 
how the motor should be set up and how operated; how 




Fig. 211. — CQ Motor installed on Ceiling. 

its direction of rotation should be changed, and how its 
speed should be varied. Considerable information is 
usually given on the name plate of the machine. If it is 
a direct current shunt motor, the name plate will state these 
facts — Direct Current Shunt Motor, giving also the voltage 
that the motor is intended to be operated upon, the speed 
of the motor at full load, the current input at full load, the 
size or capacity in horsepower, or kilowatts, and sometimes 

152 



THE SHUNT MOTOR 1 53 

the number of poles. In case the motor should not be 
provided with a name plate, it will be necessary to distin- 
guish first whether it is a direct current or an alternating 
current motor. A direct current motor always has a com- 
mutator, brushes, and separate pole pieces which can be 
readily counted, whereas an alternating current motor is a 
much simpler machine, having practically none of these 
characteristic features. This is especially true of induc- 
tion motors, although synchronous motors possess slip 
rings. When it has been decided that the motor is a 
direct current machine, the question arises of distinguish- 
ing between a shunt motor and a series motor, the two 
most common types. A series machine usually has two 
terminals coming out from the case, whereas a shunt 
machine has either three or four. There are three if the 
direction of rotation is fixed by joining together one arma- 
ture and one field terminal inside of the machine. Such 
observations apply to small machines up to 5 horsepower, 
such as a beginner would be likely to encounter. With 
larger series machines, such as railway motors, both arma- 
ture and field terminals leave the machine, as it is necessary 
to vary the direction of rotation of the motor in practice. 

Testing out Circuits. — In Figs. 21 1, 212 it will be noticed 
that four terminals come from the machine, or that the 
terminal block contains four 
outlets. Two of these out- 
lets are armature terminals, _^^^ YV x^\ ^^^^ 



and the other two are field 
terminals. As sometimes 
the character of the ter- 
minals is not indicated, the 
circuits may be tested out 
with a test lamp in the following way : 




J la 



hod 

Fig. 212. — Testing out Circuits of 
Motor. 



154 EXPERIMENTAL ELECTRICITY 

Experiment 82. Take a i6-candle-po\ver lamp, 116 volts in a socket, 
and connect one terminal to the positive terminal of a source of 120 
volts direct current, such as an Edison service. From the other ter- 
minal of the lamp run a lead to one of the terminals of the motor, 
such as terminal No. i in Fig. 212. Call this lead A. From the 
other Edison terminal extends a wire, which we will call terminal B. 
When the switch is closed, pressure will be upon A and B, and if 
brought into contact the test lamp will light. When the lamp has 
been tested in this manner, place leads A and B on terminals i and 2, 
as in «, Fig. 212, and although there is pressure upon A and B^ the lamp 
will not light, because the circuit is open in the machine. Then place 
the terminals upon i and 3, as in b^ Fig. 212, with the pressure on as 
before, and the result will be similar to that in the previous test. Now 
place terminals A and ^ on i and 4. as in <:, Fig. 212, and the lamp will 
light dimly. This is due to the fact that the field winding of the motor 
is now in series with the lamp, and as the resistance of field circuits of 
, small machines is about 100 to 200 ohms, and the resist- 

ance of a carbon filament of 1 6-candle-power hot is about 
250 ohms, the voltage across the lamp terminals will be 
/"■^N reduced proportionally. In other words, the distribution 

^\^___y\ of potejitial in a series direct current circuit is propor- 
tional to its resistance. Having located the field termi- 
\\f\J\N nals of the motor, i and 4, the terminals 2 and 3, as in d, 
Fig. 213. — Fig. 212. must necessarily be armature terminals. On 

bhunt Ma- connecting test terminals A and B to terminals 2 and 3, 
chme. . . . . . 

the lamp will light to practically full luminosity, as the 

armature resistance is very low. Where the dimensions of the field 
resistance will be of the order of 200 ohms, the dimension of the arma- 
ture resistance will be .5 ohm, including brushes. 

Experiment 83. With an ammeter and voltmeter measure the field 
and armature resistances of a shunt motor. Fig. 213, taking care when 
measuring the resistance of the armature circuit to place an additional 
resistance in series with it to protect the ammeter. 

Magnetic Circuit of Field Coils. — The field circuit of a 
motor consists of a series of poles alternately north and 
south, Fig. 214, mounted upon a frame, or yoke. The path 
of the magnetic flux is from the north pole through the 
armature to the adjoining south pole, through the yoke back 



THE SHUNT MOTOR 



155 



to the north pole. This is shown by the 
dotted Hne in the figure. The function 
of the laminated iron core of the arma- 
ture is to facilitate the passage of these 
lines of force. It is very important with 
a motor to see when connecting up the 
field coils that the polarity progresses 
regularly, namely, north and south, north 
and south. 




Fig. 214. — Magnetic 
Circuits of Motor. 



Experiment 84. Connect up the field coils 
of a shunt motor to a source of potential, the armature of the motor 
being taken from position so that coils are ac- 
cessible. Test out circuits of the excited field 
coils with a magnet or compass. If a magnet be 
not at hand, take a small bolt or a small nail of 
sufficient length to reach across the centers of 
two adjacent poles, and allowing one end to be 
in contact with one pole, bring the other near 
the adjacent pole, and if the poles are properly 
formed, the bolt will be quickly attracted to it, 
Fig. 215. 

Experiment 85. Change the order of con- 




FlG. 215. — Testing out 
Magnetic Circuits. 



verse order, and repeat the experiment. Up to the point where the bolt 
is not in contact with more than one pole, repulsion of the bolt will take 
place, but when the bolt is allowed to touch both 
poles, it will remain in contact. This is a simple 
method of testing the polarity of the field coils. 

Magnetic Circuits of Armature. — The 

armature of a motor is wound with loops 
of wire connected to the various commu- 
tator segments. When a current is passed 
through the armature, the core is magne- 
tized, forming a series of poles corre- 

. ^ ^ Fig. 216. — Testing out 

spondmg m number to the number of Ma<rnetic Circuits. 





156 EXPERIMENTAL ELECTRICITY 

field coils. Referring to Fig. 217, a core of iron is wound 
with a few turns of wire, and a current of electricity is passed 
through it. The core will then become mag- 
^'V.^'-''^ netized, having a north and a south pole. In 
an actual machine the distance between two 
des of an armature coil is equal to the dis- 
tance between the centers of two adjacent 
poles. 

^ ,^ Experiment 86. Take an armature of a 2-kilo\vatt 

Fig. 217.— Mag- ,• ^ i ^ • tr- ^• 

netic Circuits i^^chine and make a set-up, as in rig. 217, sending a 
of Armature, current of about 10 amperes through the armature cir- 
cuit. With a compass needle pass from one side of the 
armature to the other, noting the change in polarity. 

The current entering at one brush of a 2-pole machine 
passes through the winding in two directions, leaving at 
the other brush. Armature windings are quite extensive 
in number and arrangement. In many cases, however, a 
simple series winding is used, each loop being connected 
between adjoining commutator segments. 

Experiment 87. With same set-up as in Fig. 217, change the 
polarity of the terminals, alternating the direction of the current through 
the armature, reversing its north and south poles. 

Magnetic Circuits of Armature and Field. — The attrac- 
tion of the field poles for the armature poles of a motor is 
the cause of its armature rotation. With a two-pole ma- 
chine, as in Fig. 218, when the brushes are in position B, 
the motor with magnetic circuits as marked will rotate in a 
clockwise direction, the north pole of the field attracting 
the south pole of the armature. If the brushes be shifted 
over in direction A, the motor will operate in the opposite 
direction or in a counter-clockwise direction. 

Experiment 88. Excite the field coils of a small i -kilowatt machine 
and with a separate resistance in series with the armature circuit, allow- 




THE SHUNT MOTOR 157 

ing just sufficient current to pass through the armature just to overcome 
the friction of the bearings and cause slow rotation in one direction, as 
in Fig. 218. Move the rocker arm carrying the brushes, first to one 
side and then to the other, causing the direction of rotation to change. 

Neutral Plane. — When an armature coil is undergoing 
commutation, that is, when the brush is p. :ing over adja- 
cent commutator segments between which 
a coil is connected, two things happen, — ■ av/ ' J^ \ 

the direction of the current in the coil 
changes and also the coil is short-circuited. 
When the coil is short-circuited, a spark 
occurs ; this depends for its magnitude fig. 218. — Chang- 
upon the electro-motive force generated in ^"s Direction of 
the coil at that instant, upon the resistance 
of the complete circuit, including the coil and the brush, 
and upon the self-induction of the coil. The brushes are 
usually shifted to a point where sparking disappears, and 
this point is termed the netUral plane (see dotted line in 
Fig. 218). When a motor becomes heavily loaded, the 
armature flux tends to distort somewhat the field flux, so 
that the neutral plane is shifted. This necessitates shift- 
ing the brushes somewhat. 

Operating Connections. — When a shunt motor is oper- 
ating under normal conditions, the armature and field circuit 
are in multiple, as in Fig. 213, connected 
to the source of supply. Since the field re- 
sistance is comparatively high, the amount 
of current which would pass through the 
Fig. 219. — Measur- winding upon Connecting it directly to the 
ing Field Current, ^ircuit would be quitc Small. 

Experiment 89. Place an ammeter in series with a field circuit and a 
source of direct current supply, 116 volts direct current, and note the 
small current input. For a i -kilowatt machine the value will be about 
one ampere. 



116 VOLTS 

d.c. 



158 



EXPERIMENTAL ELECTRICITY 



When making connections to the field circuit, care should 
be taken to see that the main switch is open ; otherwise, 
when making the last connection, it is quite possible to 
have one hand on one terminal of the machine and the 
other hand on the other terminal of the wire which was 
about to be placed under the binding post. This will put 
the operator's body in series with the circuit, and will tend 
to make him draw back, thus opening the circuit and giv- 
ing him a shock due to the self-induction of the field cir- 
cuit. With a lo-kilowatt machine, if the machine has been 
operating and is slowing down but the starting box handle 
has not returned and the field circuit is opened through the 
body, the shock is so severe that it will be felt in the arms 
for several days. 



4. 116 VOLTS 

d.c. 



Experiment 90. Make a set-up as shown in Fig. 220, connecting 
the field terminals of the motor in parallel with a Weston voltmeter and 
a ii6-volt source of supply through a switch. The voltmeter should 
be connected beyond the switch and so connected 
that it will tend to deflect backwards, that is, the 
positive terminal of the voltmeter should be con- 
nected to the negative field terminal. Open the 
switch after the fields have been excited for a time, 
and note the kick of the voltmeter needle. This 
induced e. m.f. is due, as previously explained, to 
the self-induction of the field circuit. The flux of 
the magnetic field in shrinking to zero cuts the 
turns of wire composing the field coil and generates 
in these interlinked wires an electro-motive force, 
depending for its magnitude upon the time taken for the flux to reach 
zero and upon the magnitude of the flux. 



WV\A- 



VOLTMETER 



Fig. 220. — To show 
Induction of Field 
Winding. 



Armature Circuit. — As the armature has a low resistance, 
it is obviously unwise to connect it directly to a circuit of 
ii6-volt source as in the case of the field circuit without 
using an external resistance in series with it. 



THE SHUNT MOTOR 1 59 

Experiment 91. Having connected the field circuit to the source of 

supply, open the main switch and connect the armature ^e volts 

in parallel with it, through a suitable resistance, with an t ^•''• 

ammeter also in circuit. If the machine is a small unit, n I" 

such as a 2-kilowatt machine, a bank of lamps forms ^ — '■ 

a suitable adjustable resistance. Sufficient lamps 

should be turned on with the main switch closed. 

Fig. 221, until the motor starts to operate. If the 1'^^^''^'^''^^ 

hand be placed upon the pulley of the motor so that y l-u \I. 

it cannot rotate, the ammeter needle will deflect to a ^ Sj""v-'^ 

maximum. If the hand be removed, the motor will ^ 

, r. . ,- , 11 1 . F^G. 221. — Start- 

start to rotate, the deflection of the needle becommg -^^^ ^^ ^h.nxi\ 

smaller and smaller as the speed of the motor increases. Motor. 

Counter Electro-motive Force. — When an armature rotates 
in a magnetic field, the armature wires cut the field flux and 
thus generate an electro-motive force. This electro-motive 
force is termed in a motor a coimtei' elecU'o-motive forces 
because it tends to send a current in the opposite direction 
from the current, causing it to rotate. The terminal e. m.f. 
and the counter e. m.f. therefore oppose each other, the 
difference between the two e. m. f.'s being the e.m.f. that 
forces the current through the armature circuit. This dif- 
ference, divided by the resistance of the armature, yields 
the current that passes through it : 

F — F^ 

I=- —, 

R ' 

where / = armature current, 

R = armature resistance, 

E = line e.m.f., 

E' = counter e.m.f. 

This counter e.m.f. obviously increases with the speed, 

provided that the field strength remains constant. The 

counter e. m. f. is probably the most important characteristic 

of a motor, as it always serves to regulate the amount of 



i6o 



EXPERIMENTAL ELECTRICITY 



current passing through a motor, reducmg this current to 
a minimum and making variations of current automatic 
with changes in the load. 

Experiment 92. Continue turning on lamps as in the previous experi- 
ment, allowing the speed of the motor to increase more and more until 
finally the resistance may be reduced to zero by short-circuiting the 
lamp board. The machine will then be operating as a shunt motor, 
the armature and the field circuits being in parallel. 

As the load of a motor is increased, its speed is lowered ; 
this decreases the counter e. m.f., which in turn allows more 
current to pass through the armature. As the armature 
current increases, the torque or twisting force of the axle 
increases. This process of adjustment is going on con- 
tinually in a shunt motor, the input always equaling the 
mechanical output in horsepower plus the losses in the 
machine. 

Starting Boxes. — The function of a starting box is to reg- 
ulate at starting the input of current into the armature cir- 
cuit. The starting box for a shunt motor usually has three 
terminals, although a new form of four-terminal box is being 
placed on the market by the General Electric Company. 
The circuits of a starting box may be most readily under- 
stood by referring to an old form of 
-^^ NX starting box. Fig. 222, made by the 

^W_ \\ Crocker Wheeler Company. The 
> T2) ) starting box has three terminals 

marked Z, line. A, armature, and F, 
field. The moving arm is connected 
through a brass strip to the terminal 
FIG. 222. -starting Box. ^^ ^^ j-^^^ ^j^^^ ^j^^ handle is 

moved to the first position, it makes a contact at one end 
with another brass strip connected to the terminal F, or field. 
With a continued motion of the arm it continues to make 



THE SHUNT MOTOR l6r 

contact with the field strip and it also makes contact at its 
other extremity with a series of individual brass contacts 
which are connected inside of the box to a resistance which 
terminates at A, the armature terminal of the box. By 
still further continuing the motion of the arm 
these armature contacts are passed, one at a 
time, until at the end of the travel the arm 
makes contact directly upon the armature con- 
tact A. When the starting box arm is on 
the first contact, field circuit current only 
passes through the starting box. When on 
the first armature resistance, the line current 
also passes through the armature circuit, being 





limited in magnitude by the series resistance Fig. 223. -Start- 
in the box. At the end of the travel of the ^"s ^^^^ ^^^^ 

On. 

startmg box arm, the armature resistance has 

been eUminated, and the armature and field circuit are in 

shunt with each other. Fig. 223. 

Directions for connecting up a Shunt Motor. — Be sure 
that oil has been placed in the bearings, that brushes are 
in contact with the commutator, that the armature can rotate 
freely, and that there are no loose connections. 

Experiment 93. Referring to Fig. 223, connect to the service — 116- 
volt direct current — a double pole switch of proper current-carrying 
capacity. Connect one armature and one field terminal of the motor 
together, and connect the junction to one of the terminals of the switch, 
Fig. 223. Connect the other terminal of the switch to the line terminal 
L of the starting box. Connect the armature terminal A of the start- 
ing box to the free armature terminal of the motor, and also connect the 
F, or field terminal of the starting box, to the free field terminal of the 
motor. Close the main switch and turn the starting box handle to first 
contact, thus exciting the field circuit of the motor. Attempt to turn 
the motor armature while the field circuit is excited, and notice the 
stiffness of the field. Turn the starting box handle back to zero, and 
notice the arc due to opening the field circuit. Turn the handle on 



1 62 EXPERIMENTAL ELECTRICITY 

again and continue the motion " slowly/' allowing the speed of the motor 
to increase gradually until a maximum position is reached. 

Magnet Arm on Starting Box. — With the old form of 
starting box just described there was danger of some one's 
opening the main switch and forgetting to turn back the 
starting box arm. When it became necessary to start the 
motor again, the main switch was closed without looking 
at the starting box, and as a result the fuses were blown, 
as a short circuit through the armature circuit was formed. 

To eliminate such occurrences 
various manufacturing compa- 
nies, such as the Cutler Hammer 
Manufacturing Company, devel- 
oped the form of starting box 
shown in Figs. 224, 225. In this 
the forward motion of the arm 
was resisted by a spring, which 
would return the arm to zero po- 
sition if released. When the 
Fig. 224. — Starting box. (West- arm, howevcr, is carried over 
ing ouse.) until it reaches the magnet. Fig. 

225, it is held there as long as the magnets are energized. 
The winding of this magnet is connected internally in the 
box so that it is in series with the field winding of the 
motor. If it is desired to stop the motor from operating, 
the main switch is opened. As the armature of the motor 
continues to operate, due to its inertia, it generates an 
electro-motive force which sends a current through the 
shunt-connected field circuit and helps to maintain the field 
excitation. When the speed of the motor has decreased 
sufficiently so as not to endanger the motor should the main 
switch be thrown, the current in the series magnet becomes 
weakened, and the spring throws back the starting box arm. 




THE SHUNT MOTOR 



163 



Caution, — In order to stop a motor do not "knock" 
back the starting box arm ; instead, open the main switch, 
since otherwise the e. m. f. of self-induction of the field 
circuit may puncture the field winding or the insulation of 
the adjoining wires in the starting box. The writer has had 
occasion to repair both these types of injury due to this 
cause. Furthermore, look always at the starting box arm 
before closing the main switch in order to be sure that the 
arm has not " stuck," as occasionally happens. This mag- 
net arm is sometimes called a low voltage release, for if the 
voltage on the system becomes low temporarily, the magnet 
would not be sufficiently energized, and the arm would fly 
back. 

Overload Release. — Some starting boxes are equipped 
with overload release. Fig. 225. This release consists of a 
coil connected in series 
with the main line cur- 
rent going to the motor. 
When the current be- 
comes excessive, a 
hinged armature is 
drawn up having two 
copper strips mounted 
upon it. When these 
two contacts, electrically 
connected, are drawn 
up, they short-circuit 
the metallic uprights 
connected to the ter- 
minals of the retaining ^^^- ^^5- - Cutler Hammer starting Box. 

magnet, and allow the magnet arm to fly back. 

Experiment 94. Make regular set-up with a starting box provided 
with overload release. Put a brake load upon the motor, and note the 




1 64 



EXPERIMENTAL ELECTRICITY 



operation of release. Also, trip coil by raising its armature with the 
finger. 

Note. The armature of the release may be raised so as to vary its 
distance from the pole of its solenoid ; in other words, it may be " set " 
for various overloads. 

It is quite important not to start a motor too quickly by 
moving the starting box arm over too fast, as the inrush of 
current may be so great as to blow out the main fuses. 

Experiment 95. Connect an ammeter, projecting, in series with a 
regular shunt motor, and turn on the handle of the starting box at vari- 
ous rates to determine the range of the starting current. 

Changing Direction of Rotation. — In order to change the 
direction of rotation of a shunt motor, it is necessary to 
change the direction of the current, either through the 
armature or the field circuit. 

Experiment 96. Make an ordinary set-up of a shunt motor, as indi- 
cated in Fig. 226 a. and note the direction of rotation. 




d.c. 






116 VOLTS 



A fL 



Fig. 226. — Motor Circuits. 

Experiment 97. Without disturbing the line connections, disconnect 
the junction of the armature and field circuit at 2-3, Fig. 226 a, and dis- 
connect the armature terminal A from the starting box. Connect field 
terminal 2, Fig. 226 b, to armature terminal 4, and connect the junc- 
tion to the service wire. Connect armature terminal 3 to the starting 
box at A. This will reverse the direction of the current through the 
armature circuit. Fig. 226 b, and will change the direction of rotation of 



THE SHUNT MOTOR 



165 



the machine without changing the previous direction of the current 
through the field circuit. 

Experiment 98. Connect armature terminal 4 to the field terminal 
I and connect to service, as in Fig. 226 c. Connect armature terminal 
3 to the starting box and field terminal 2 to starting box. This will 
maintain direction of current through armature the same as in previous 
experiment, but will reverse the direction of the current through the 
field circuit, changing the direction of rotation of the armature, Fig. 
226 c. 

Experiment 99. Change over the line terminals at the switch, ex- 
changing the positive for the negative, and note that the remainder of 
the set-up is undisturbed. Direction of rotation will remain unchanged. 

Speed Variation. — A variation of speed 
in a shunt motor may be obtained by 
shifting the brushes, by placing a resist- 
ance in series with the armature circuit, 
or by placing a resistance in series with 
the field circuit, Fig. 
227. The method usu- 
ally employed in prac- 



Cl o 

OA o 



L-WAAAAA/' VVW 




RHEOSTAT tice is placing 
Fig. 227.-Fieid Circuit, the resistance 
in series with the field circuit. 




2 




Fig. 228. — Remote Control Resistance. 



1 66 



EXPERIMENTAL ELECTRICITY 



Figure 228 shows the method of instalHng large rheostats 
where the switch is separate from the resistance (G. E. Co.). 
The value of this resistance should be about equal to that 
of the field resistance of the motor, so that the field current 
can be reduced to half value. 

Experiment 100. Make a set-up, as shown in Fig. 227, placing the 
field rheostat in series with the field coils of the motor. Vary the resist- 
ance, and note the change in speed. 

Note. Never start up a motor without being sure that the field 
rheostat handle is at zero or that all of the resistance is out of the cir- 
cuit, otherwise the motor will accelerate too quickly. With a weakened 
field there is a lower counter e.m.f. induced in the armature at starting, 
and consequently there is a greater inrush of current than necessary, 
causing too rapid an acceleration. 




Fig, 



229. — Self-contained Rheostat (G. E, Co.) 



Take care not to open the field circuit of a motor while operating, as 
the motor armature may " run away," for with almost a zero field it 
would require an infinite speed to generate sufficient counter e.m.f, to 
limit the armature current. For small size motors, rheostats are usually 
self-contained, as in Fig, 229, 

How to tell whether Field Resistance is All In or All Out. — 

Rheostats when mounted upon a switch board are usually 
marked ''resistance out," ''resistance in," indicating the 
direction of motion to produce one condition or the other 
by arrows, see Fig. 230. The resistance is connected 



THE SHUNT MOTOR 



167 





Fig. 231. — Cir- 
cuits of Rhe- 
ostat. 



Dial 
of Rheostat. 



between two terminals of the metal support as in Fig. 231. 

An additional wire or metal strip connects one 
of the terminals to the moving 
contact arm. As this arm is 
moved, it short-circuits part of the 
resistance of the box. When the fig. 230. 
arm is to the right, Fig. 231, all 
of its resistance is in circuit, and when it is to 

the left, all of the resistance has been eliminated. 

Field rheostats are made in very compact form for small 

machines by the Cutler Hammer Company, as 

shown in Fig. 232, the rheostat being filled 

with porcelain as shown in Fig. 233, in which 

the resistance is embedded. 




Field 



Experiment loi. Take a 500-ohm rheostat, and 
place it in series with a i6-candle-povver lamp and -t'^'^-^S^ 
a 120-volt direct current circuit. Vary the rheostat 
and note its position when resistance is '*all in,'' and when it is "all 
out."' When in doubt, look at the back of the rheostat. 



COMMUTATOR 




Fig. 



233- 



Cross Section of Cutler Hammer Rheostat. 



Experiment 102. Use a field rheostat which is 
open circuited, burnt out. as in Fig. 234, and make 
a set-up similar to that in the previous experiment. 
Use a short piece of wire termed a jumper and 
Fig. 234. — Burnt-out make a contact between the various terminals until 
Rheostat. the lamp lights, showing that the jumper has 




1 68 EXPERIMENTAL ELECTRICITY 

spanned the gap. Solder this jumper to these two contacts, taking care 
that the movable arm can slide over the soldered connections unob- 
structed. The rheostat has now been repaired. 

Theory of Speed Variation. — When the field circuit of 

a motor has been weakened by the insertion of resistance, 

the armature counter e. m. f. will be correspondingly 

decreased. When this happens, according to the law 

E — F' 
I = —, the armature current will increase as E and r 

r 

are constant. As the current in the armature increases, 
the torque increases, and this increases the speed. As the 
speed increases, the counter e. m. f. increases, until, as 
previously stated, the energy input equals the output plus 
the losses in the machine. 

Location of Trouble. — When a motor does not operate, it 
is not necessarily disabled. It may be improperly set up, 
there may be terminals loose, brushes may be not in con- 
tact with the commutator, or something may be the matter 
with the starting box. The following procedure is desir- 
able in case the motor will not start : 

1. Spin motor armature with hand, and see that the 
armature is free. 

2. See that the brushes are in contact with the com- 
mutator and that oil is in the bearings. 

3. Go over the machine carefully, and see that all wires 
in the set-up have been properly connected. 

4. Test out the circuit with fingers or with lamp, and 
see that the power is " on." 

5. Put the starting box handle on the first contact, clos- 
ing the field circuit, and allow it to return to zero. If the 
switch flashes as it leaves contact, the field circuit is prob- 
ably satisfactory. If no flash occurs, the field circuit is 
open ; it may be open in the starting box or it may be 



THE SHUNT MOTOR 1 69 

open in the motor. Some field terminals are very poorly 
connected in the machine, and they,, have a tendency to 
become loose. 

6. If the field circuit is satisfactory, continue the motion of 
the starting box arm so that contact is made with the arma- 
ture circuit. If the motor does not start, the armature cir- 
cuit may be open. If the motor runs " wild " as the starting 
box handle is turned on and the starting box begins to 
smoke, it is a pretty sure sign that the field circuit is open. 

7. When the machine is operating, if the commutator 
flashes as it goes around, this may be the result of a high 
bar, a short-circuited armature coil, or a hard spot in the 
brushes. 

8. If the field coils heat excessively, if the sparking of 
the brushes becomes severe, or if the armature smokes, 
there are defects in the machine which must be remedied. 
The fault may be located in some of the following ways. 

Tests for Grounds. — A simple test lamp connected up 
with one terminal to the service and the other terminal 
connected to the fraaie of 
the motor, as in Fig. 235, 
and an extra test wire ex- 
tending from the other ser- 
vice terminal which can be 

connected to various parts of ^IG. 235. -Test for Grounded 

Armature. 

the machine, are all that is 

necessary for ordinary ground testing. Where it is desired 
to measure the resistance of the ground, a 150-volt volt- 
meter substituted for the lamp will yield the correct result. 

Experiment 103. Test for grounded a7'mature. Make the set-up as 
shown in Fig. 235. raise the brushes so that they will not be in contact 
with the commutator, and place terminal B on the commutator. If 
the lamp lights, the armature is grounded. All of the armature wires 
should then be unsoldered and disconnected from the commutator, and 




I/O EXPERIMENTAL ELECTRICITY 

terminal B should be placed in contact with the commutator. If the 
lamp lights, the commutator is grounded. Place terminal B on the 
armature winding after testing the commutator and make a similar test. 
When disconnecting the wires from the commutator, they should be 
tagged so that they can be properly replaced. 

Experiment 104. Grounded Field Coil. — Disconnect the field ter- 
minals from a terminal block and placing termi- 
nal B, Fig. 236, on the field terminal, note result. 
The field coils may be easily disconnected from 
each other, so that a grounded field coil may be 
readily located. 

Fic;. 236. -Test for Tcsts for Short Circuits. — A short- 

circuited coil is more difficult to locate 

than a grounded coil. With a four- ,,^ ^^^^ . 

pole machine, if one of the field coils ^'''' ^■pvJrJvv^r-w^^r-^w— ' 
be short-circuited, the coil may be lo- 




cated with a voltmeter as in Fis^. 237. ne volts" , , 

The resistance of the various colls are "■ ^ I^J-^^^^^ 
approximatelyequal, and consequently fig. 237. — Test for Short- 
the distribution of potential is propor- circuited Field Coiis. 
tional to the resistance, the short-circuited coil having a 
low voltage reading. 

Experiment 105. With a four-pole machine connected to a ii6-volt 
direct current source of potential, as Fig. 237, 
measure with a voltmeter the distribution of 
potential across each field coil. 

Experiment 106. With same set-up short- 
circuit one of field coils. Fig. 237, and notice 
the readjustment of potential. In the first 
Fig. 238. -Test for Sh^rt- case the volts per coil will be 29 on a ii6-volt 
circuited Armature circuit and in the second case the volts per 
Coils. coil will be 38.6. 

Short-circuited Armature Coil. — A short-circuited arma- 
ture coil requires more care in its location than a similar 
defect in a field coil. 




THE SHUNT MOTOR 171 

Experiment 107. Connect a resistance in series with the brushes of 
a short-circuited armature and a 120-volt circuit so that about 5 amperes 
is passing through the circuit. Test with a low scale voltmeter across 
each pair of adjoining commutator segments, Fig. 238. A low reading 
of the voltmeter will indicate trouble in the coil. Be sure that the main 
current is passing through the armature at all times while the test is 
being made, for if the circuit should be open in two coils, one on each 
side of the brushes, a low voltage voltmeter would span the gap while 
testing, and the needle would be badly jammed by having 120 volts 
across the low voltage, 3 volts, coil. 

Experiment 108. Prepare an old armature with two open-circuited 
coils and a short-circuited coil. Make a set-up similar to that in the last 
experiment, and test with the voltmeter. Use 150-volt scale of the 
voltmeter so that when the voltmeter spans open circuit and the entire 
potential is across the voltmeter, the needle will not bank, as it would 
if low scale were used. 

Resistance of Ground. — A standard Weston voltmeter 
with a high resistance of about 15,000 ohms serves as a 
ready method of testing for grounds. By placing the 
voltmeter in series with a source of potential and then by 
testing one terminal of the device, the resistance can be 
readily calculated as follows : 

Experiment 109. Take a switchboard which is partially grounded, 
and make a set-up as in Fig. 239. Connect one ter- 
minal of the voltmeter to the positive switch and the — "« ^olts 4. 
Other terminal to the metal part of the board. The X 
resistance is then calculated by reading the voltage of par- 
the service, the amount of drop in voltage, and using 
the proportion, 

e:e' ::R:x, ^^^ 2^^_ _ ^^^^ 

where e = voltmeter reading when drop in voltage e' of Grounded 
is obtained, and R — resistance of voltmeter. Switchboard. 

E = e -\- e' . 
The distribution of potential is proportional to the resistance in a series 
direct current circuit. 



1/2 



EXPERIMENTAL ELECTRICITY 



Interpole Motors. — The introduction of interpoles in 
motors, Fig. 240, has resulted in improving their commu- 
tation to a marked extent, so much so that a motor can be 




Fig. 240. — Interpole Motor (G. E. Co.). 

reversed at full speed. The interpole winding is usually- 
placed in series with the armature and tends to neutralize 
its self-induction. 



QUESTIONS 

1. Explain how you would tell a shunt motor from other motors if 
the machine did not have a name plate. 

2. What is the function of a starting box and how would you set up 
a shunt motor with a starting box ? Give diagram of connections. 

3. How would you change the direction of rotation of a motor ? 

4. If motor sparks as it rotates, how would you go about locating the 
trouble, if, after cleaning the brushes, and the commutator, and shifting 
the rocker arm, it still continued ? 

5. How would you determine whether field coils were grounded, 
short-circuited, improperly connected, or burnt out ? 



THE SHUNT MOTOR 173 

6. What do we mean by counter e.m.f., and how does it affect the 
speed variation of a shunt motor? 

7. How would you locate a burnt-out coil in an armature ? 

8. Why is the shunt motor essentially a constant speed machine? 
Give theory of speed variation. 

9. If an armature was operating on a ii6-volt circuit, the resistance 
of the armature was .10 ohm, and 10 amperes were passing through the 
armature, what would be the counter e.m.f. ? Ajis. 115 volts. 

10. What is the cause of the heavy spark on breaking the field cir- 
cuit of a motor, although the current passing through the field circuit is 
very small ? 

11. In what manner do the interpoles on a motor eliminate spark- 
ing ? 

12. If 746 watts equal a horsepower, approximately, how many am- 
peres can you figure to the horsepower for a motor on a ii6-volt circuit ? 

A71S. 6.43. 



CHAPTER X 
THE SERIES MOTOR 

Characteristics of the Series Motor. — In general appear- 
ance and construction the direct current series motor is 
similar to the direct current shunt motor discussed in the 
last chapter. A series motor possesses an armature, a 
field circuit, brushes, commutator, and many other char- 
acteristic features of the shunt motor. 

The counter e. m. f . generated in the armature of a series 
motor plays the same important part in the operation of 
the motor as it does in the shunt motor, and the armature 
also requires some form of starting device. The direction 
of rotation of the series motor may be changed, as with 
the shunt motor, by changing the direction of the current 
through either the armature or the field circuit. 

From an operating viewpoint, however, the series motor 
differs from the shunt motor, being a variable speed 
machine, whereas the shunt motor is a constant speed 
machine. The field circuit of a shunt machine has a 
constant excitation, whereas the excitation of the series 
motor is variable, all of the current which passes through 
the field circuit passing through the armature circuit, as in 
Fig. 241. The following suggestions may be of interest to 

eoo VOLTS D.c. those who find it necessary to distinguish a 

I I series motor from a shunt motor. A series 

L^->_^^^,^, motor, if its direction of rotation is fixed, 

Fig. 241. — Series usually has two terminals on the frame of 

Motor Circuit the machine instead of three or four, as with 

the shunt machine. This does not apply to railway motors, 

in which it is necessary to change the direction of rotation. 

174 



THE SERIES MOTOR 1 75 

As the same current which passes through the armature 
circuit passes through the field circuit also, the field wind- 
ing as well as the armature winding must have a low re- 
sistance in order to keep down the resistance losses. The 
field windings are therefore wound with copper strip of 
many times larger cross section than the field winding of a 
shunt machine. 

Magnetic Circuits of the Series Motor. — The magnetic 
circuits of a series motor are similar to those of the shunt 
motor, and the same tests may be applied to determine 
whether the field coils are properly connected, and also to 
locate grounds and open circuits in the windings. 

Resistance of Armature and Field Circuits of the Series 
Motor. — As the same current which passes through the 
armature circuit of a series motor passes through the field 
circuit, the field coils resistance must be low, as previously 
stated, in order that the energy lost in overcoming the re- 
sistance of the field winding will not be excessive. 

Experiment no. — Measure resistance of armature and field circuits 

of a series machine, taking the precaution to place a + -■ 

series resistance in circuit with the machine, as indi- |«^ (^ 

cated in Fig. 242. If the current used in this experi- J 

ment is sufficiently low, the machine will not rotate. 

If it does tend to rotate, the armature should be blocked, j^-iT^'^^C""^ 

for if voltage readings are taken across the armature Kv)-' Ky)-' 

circuit while rotating, the voltage value indicated will Fig. 242. — Meas- 

be a resultant of the counter e. m. f. and the IR drop. "^ing Resist- 

The magnitude of the field resistance will be of the f''''^' °^ ^^"^^ 

^ , r , • , . Motor. 

same order as that of the armature resistance, that is, 

about .5 ohm for small machines and .05 ohm for large motors of 

150-200 horsepower as used for railway operation. 

Starting of a Series Motor. — Since the counter e. m. f. 
developed in the armature of the series motor performs 
the same function as in the shunt motor, and since the 



Ije EXPERIMENTAL ELECTRICITY 

armature and field resistance of the motor are both of 
small magnitude, a series starting resistance becomes a 
necessity. This resistance is placed directly in the circuit, 
and in series with both the armature and the field circuit. 
It will be remembered that with the shunt machine the 
resistance was placed simply in the armature circuit. As 
the speed of a series motor increases, this resistance is 
gradually eliminated. The counter e. m. f. increases with 
an increase in speed, and this tends to lower the current 
input. As the current input decreases, the field strength 
decreases, and this tends still further to increase the speed. 
This process of adjustment continues while the machine is 
in operation, and requires that where the resistance values 
of the motor are low, the motor's load should be connected 
to it rigidly. The series motor is used only where it can 
be directly connected to its load, as in railway operation 
and in elevator work, in which a large starting torque is 
desired. The speed of the motor governs its energy input, 
the energy output plus the losses equalling the energy 
input. There are in use series motors of small capacity 
and of large resistance which may be directly connected to 
a 240-volt service without resistance in the circuit. 

When the current increases with decreased speed, the 
torque, or twisting force, of the armature increases, the 
motor exerting its greatest tractive force at zero speed. 
With railway operation this increased tractive effort at low 
speeds is especially desirable, as it is necessary to over- 
come the inertia of the train at rest. When the train is 
operating at maximum speed, the energy consumption is 
reduced to a minimum. After a time the power is cut off, 
and the train is allowed to coast, the brakes being finally 
applied. About 65 % of the energy put into the train 
while accelerating is taken from the train at braking. 




THE SERIES MOTOR 1 77 

Speed and Tractive Effort Curves. — It is customary to 
show the performance of a series motor by means of a 
series of curves, all plotted in terms of current input. 
These curves indicate speed, tractive effort, and efficiency 
at the various current values. The term t7'active effort, 
when applied to a railway motor, means the horizontal pull 
at the base of the car wheel, Fig. 243. The term torque 
means the moment of pull on the shaft of 
a motor and is usually expressed in foot- 
pounds. The pull on a belt passed over a 
pulley expressed in pounds, multiplied by ^^^ 243.-Trrtive 
the radius of the pulley in feet, will give Effort of Series 
the torque or twisting force of the motor Motor. 
for that particular radius of pulley. With a trolley motor 
a small gear mounted upon the shaft of the motor meshes 
into a larger gear on the wheel axle of the car. One side 
of the motor is upon the car axle, and the other side of the 
motor faces the other car axle. Upon the latter is mounted 
-loji^!^. t^^ larger gear, sus- 

©^ — ix*^^^"* pended by various forms 
A/^'^A of suspension called bar, 
^^^sLJ\ J nose, or spring suspension, 
PINION X ^ ^ . T RACTIVE, , ,. , 
AXLE wHEEL^^ ^EFFORT dcpcndmg upon the man- 

FlG.244.-Relationof Torque to Tractive ^^^ '^^ ^\{xq\v it is aCCOm- 
Eifort of Series Motor. , . , ^ ^ 1 . 1 •, 

plished. It may readily be 
seen, then, from Fig. 244, that the torque of the motor may 
be converted into tractive effort, the tractive effort, as 
previously stated, being the horizontal pull at the base of 
the car wheel. 

In the characteristic curve sheet. Fig. 245, it may be 
noted that the tractive effort is almost proportional to the 
current input, except at the lower part of the curve below 
the knee of the magnetization curve or where the iron is 
below saturation. 



1/8 



EXPERIMENTAL ELECTRICITY 



The tractive ejfort of a series motor depends solely upon 
the current input, to which it is directly proportiotiaL It is 



1 


J 

i 

56 
52 


I 

4) 
> 

i 




48 


2400 




44 


2200 


00 


40 


2000 


90 


36 


1800 


eo 


32 


1600 


70 


26 


UOO 


60 


24 


1200 


60 


25 


1000 


40 


l€r 


eoo 


30 


12 


600 


go 


6 


40'0 


10 


4 


200 












75 HP output at 130 Amp-rnpCft. 

Volts at motor terminals 500. 

Diamieter of car wheel 33 " 

Armature Z turns. 
PinionFA, Gear 51 Ratio Z. \Z 







1 1 


j 
















t 












\ 












i 












i 










7 




\ 








■ 7 




\ 








_ / 
















\ 








/ 










Eff.c.encv " 


7 




K 






/ 


— — . 




/ - 














\ 




7 ' 




/ 








7 : 




r 




















/^-^ 


Snf>^- 




















.- 


/ 


■* 








^V 










<2ty 1 




















■ A 


/ 










l> 












/ 












^ 










y 












/ 








GE 


;-73-C-l2 



Fig. 



20 .40 60 80 100 J20 UO 160 I80_2Q0 220 
Amperes 
245. — Characteristic Curves of Series Motor. 



also independent of the line voltage for the same ciirre7it 
input. 

The speed of a series motor is approximately inversely 
proportional to the square of the current between certain 
limits. If the current input is at a minimum, the change in 
speed is very great for a small change in current ; but if the 
current input is near the full load value, a large change in 
current can occur with a small change in speed. 



THE SERIES MOTOR 



179 



The speed of a series motor for a given citrrejtt inpiU 

varies with the impressed voltage. 

The e. m. f. current relations in a series motor are 

E — E' 
expressed by the formula /= , where / is the 

current passing through the armature, E is the impressed 
voltage across the armature terminals, E' is the counter 
e. m. f ., and r is the resistance of the armature circuit. 
Characteristic curves. Fig. 245, are always taken for a 
constant impressed e. m. f., whose value is usually given 
on the curve sheet. 

Experiment m. Place a projecting ammeter in series with a small 
series motor, and place a brake over the motor pulley, so arranging the 
brake that its motion may be 

I' 



magnified as in Fig. 246 in order 
that a large number of people 
might be able to see such motion 
as would occur. Take a series of 
observations of pounds pull for 
various current values, and plot 
curve showing current torque 
relations (lecture experiment). 

Experiment 




SCALE TO 
MAGNIFV MOTION 



FIC 



246. — Measuring Torque of Series 
Motor. 



116 VOLTS D.C. 



U^^ 



WESTON SPEED 
TACHOMETER 




12. Witli the same motor set-up belt a Weston speed 
tachometer to the motor pulley and connect 
electrically the tachometer to a projecting 
vokmeter. Fig. 247, calibrating the vohmeter 
to read speed (that is, so many revolutions 
of the motor, as shown by a speed indicator, 
would be equivalent to so many volts gener- 
ated). Vary the speed by placing a light 
load upon the motor, and take a series of 
observations of speed and current input. Arrange a double throw 
switch. Fig. 248, so that the projecting galvanometer terminals come 
to the middle terminals, and so that two of the switch terminals are 
connected to the ammeter shunt, and so that the other two switch ter- 
minals are connected to the speed tachometer through a series resistance. 



m 



Fig. 247. — Measuring Speed 
of Series Motor. 



WESTON MAGNETi 
TACHOMETER 




1 80 EXPERIMENTAL ELECTRICITY 

Readings of current may then be taken by throwing the switch in one 
direction, and readings of speed may be taken by throwing the switch in 

the opposite direction. A ^-horse- 
power motor is very satisfactory 
for this experiment (lecture ex- 
periment). 

Changing Direction of Ro- 
tation of a Series Motor. — 

Fig. 248. — Showing Relation of Speed to The same method may be 
Current. used to change the direction 

of rotation of a series motor as a shunt motor, namely, 
changing the direction of the current either through the 
armature or through the field circuit. In railway motors it 
is customary to change over the armature terminals. 

Experiment 113. — Set up a small series motor, Fig. 249, and change 
the direction of rotation by changing over first the armature and then 
the field terminals. * 

Railway Controllers. — In starting a series motor an 
auxiliary series resistance is a necessity. This resistance 
is gradually eliminated from the circuit as the 
speed of the motor increases. The device Lq— v^v^^J 
used in railway work to accomplish this func- ^ _ 

tion is termed a co7ttrolle7\ Controllers are so LtJqI j 

arranged that the direction of motion of the 

car may be changed also by the manipulation .+ | -, -1 

of a handle separate from the main controller ^^ 
handle, or the circuits may be arranged so ^c^uitstordiang- 
that when the controller handle is moved in ing Direction 
the opposite direction the car's motion is re- °^ Rotation of 

. . Series Motor. 

versed. The latter method is used in the 
equipments of the Westinghouse Manufacturing Company. 
Controllers are of two fundamental types, namely, hand 
control and automatic control. Both types of control 
require the manipulation of a main controller handle by 



THE SERIES MOTOR l8l 

a motorman. The motion of the main controller handle 
gradually eliminates the resistance from the circuit. With 
the automatic control a limit switch, page 29, automatically 
permits the forward motion of the controller, the limit 
switch being in series with the main line. 

As the current which is interrupted at the various 
notches in the manipulation of a controller is of large 
magnitude, both in the hand control and the automatic 
control, means must be taken to extinguish the arcs formed. 
This is accomplished by magnetic blow-outs described on 
page 34. The principal type of controller in use is known 
as the series multiple control. The pur- + - 

pose of the series multiple control is to L-O-^vvvv-rv^/s/vJ 
provide two or three running positions ^^^ ^so.-Two Motors 
for the motors. A running position, in Series. 

Fig. 250, is one in which all of the external resistance has 
been eliminated from the motor circuit, the resistance grids 
- not being intended for continuous operation. 
With a two-motor equipment of series mul- 
tiple control, as in Figs. 250, 251, there 
Fig." 251. — Two would be two running positions possible. In 

Motors in Par- scrics position on a 600-volt direct current 

allel 

circuit there would be 300 volts across each 
motor, the motor operating at half speed, and at full multi- 
ple position, or 600 volts full speed, both motors would be 
operating in parallel on 600 volts. With 
a four-motor equipment three running Lo^v*c» — o-^n^ 
positions may be obtained. In one case F1G.252. — Four Motors 
the four motors are in series with each 
other. Fig. 252, having a potential difference of 150 volts 
per motor. In the second position the motors are connected 
in series of two sets in parallel having a potential difference 
of 300 volts per motor, Fig. 253. In the third running 




l82 



EXPERIMENTAL ELECTRICITY 



position the potential difference is 600 volts per motor, 
Fig. 254. With a four motor equipment such as the four 40 
horsepower motors used in trolley opera- 
tion, it is customary to employ the series 
multiple control 
having two running 
positions. During 



Fig. 253. — Four- 
motor Equipment. 



SERIES 1st POINT 

-p MOTORS 



p^^w-OJ-j 



the last few years ^i^^'^^^'^^^^^^'^^j^ 
there has been in- full series a 

troduced a modifi- 'L^^^j.^^.X^:;^^ 
cation of the ordi- 5432 



Fig. 254. —Four Mo 
tors in Parallel. 



-^^/l^-(2)-JqjnlqfdrijiIr^ 






PARALLEL Ut POINT A 



H 




nary controllers 

which employs fullseriesb 

some of the features of the auto- 
matic control. These features con- 
sist in the use of a few contactors 
placed under the car so as to open 
and close the trolley circuit away 
from the main controller. The pur- 
pose of this is to eliminate the so- 
called "controller burn-outs." A 
series of experiments illustrating the 
action of controller burn-outs are 
recorded by the writer in the Engi- 
neers' News. In the old types of 
controller it was customary in pass- T^ ;i/i/if!^ Jr, Jr^r. . "JQ 

ing from the series to the multiple ^ 4 32 1 

position to open the controller cir- ^•WKh^LopMoJ^^ 
cuit, thus causing an unwelcome Fig. 255. - Bridge Control. 
jerk to the passengers in the car. This is now avoided in 
the automatic control by what is known as the bridge con- 
nection, Fig. 255. The principle of the bridge connection 
is that the two motors are connected in series with each 



PARALLEL 1st POINTS 

I<T)^VI^-'in-'ip-n_n — 3- 

rJ\/^^L(D — qjTJin-TJ-ilrijulL 
FULL PARALLEL , 



THE SERIES MOTOR 1 83 

Other, with resistance, and the Kne voltage. The resistance 
is gradually eliminated from the circuit by means of con- 
tactors until both motors are in series with the 600-volt cir- 
cuit. Full series B. 

The ground switch and positive switch are now con- 
nected, the short-circuiting switch is still closed. This 
places both motors in parallel with each other and the 600- 
volt circuit and with resistance, the bridge connection still 
remaining. The switch a is then opened, and the series 
resistance eliminated step by step from both motors until 
they are both operating in parallel with the 600-volt cir- 
cuit. A complete description of the various methods of 
control with detail diagrams may be found at pages 102 to 
164 of the author's Electric Railways, Vol. I. Space will 
not permit a fuller description here. 

Emergency Brake of the Series Motor. — In case the air 
brakes of a car should fail in their operation, it is well to 
know how to stop the car by means of the motor. With 
series motors operating with field circuit excited, if the 
power be turned off when the car is operating at a fair 
speed, the reverser on the controller thrown, and in a two- 
motor equipment the controller handle turned to full mul- 
tiple position, the motors will become generators operating 
in parallel, the one generating the higher e. m. f. sending its 
current through the other motor, causing it to motorize and 
tend to change the direction of rotation. With a four- 
motor equipment where only two running positions are in 
use, the motors being operated in two pairs of two motors 
which are permanently connected in parallel, it is simply 
necessary to open the line switch and throw the reverser, 
the car coming to a stop almost instantaneously. 

Experiment 114. — Take two series motors mounted upon a stand 
equipped with type H controller and practice the emergency brake. Do 



1 84 EXPERIMENTAL ELECTRICITY 

not allow the speed of the motors to become too great lest the armatures 
may become injured by being raised against the field coils. Place an 
ammeter in the armature circuit of one of the motors, and notice the 
large magnitude of the current passing through the motor when the 
braking effect takes place. 

Testing out Controller. — In order to minimize the trouble 
from defective controller wiring or defective insulation of 
controller, it is well to subject the controller equipment to 
two tests. One of these tests should be made at fairly 
frequent intervals ; this is the regular voltmeter test (page 
128) which measures the insulating value in ohms of the 
resistance. The second test is the use of at least 2200 
volts, and preferably 6000 volts, alternating e. m. f. from a 
transformer, one terminal being grounded, and the other 
terminal placed" on the line controller leads and ground 
connection of the motor removed. This high voltage 
will break down defective insulation. In the operation 
of electric trains or trolley cars it is well to guard not 
only against controller burn-outs, but also against delays re- 
sulting from breakdowns. 

Structural Features of the Series Motor. — Owing to the 
heavy service imposed upon series motors, particularly in 
railway operation, it is necessary to construct them some- 
what differently from ordinary series motors. They have 
one advantage over the stationary motor when mounted on 
a car in that they will carry about 25 % more load owing 
to their better ventilation. Railway motors must be com- 
pact, as the space in which they are installed is quite 
limited. Their frames must be arranged either upon 
hinges to allow the armatures to be taken out or, as with 
the box frame type of motor, with the end castings that 
hold the armature in position so placed that the armatures 
may be slid out, these obviating the necessity of using a 
pit. The motors must be inclosed so far as possible, since 



THE SERIES MOTOR 1 85 

frequently they have to run through water which comes 
part way over the field frame. The armature windings 
must be cross connected so that with a four-pole machine 
only two sets of brushes will be necessary, and these brushes 
may be reached through the trap in the car floor. 

In all cases where is it necessary to go under a car which 
is either in the shops or on the main line, care should be 
taken to display proper signals to indicate that a man is 
beneath the car, and when the work is being done in the 
shops, an " out of service " sign should be used in addition. 



QUESTIONS 

1. How does a series motor differ in structural features from a shunt 
motor ? 

2. What classes of service is the series motor especially adapted for 
and why ? „^ 

3. Explain how theT:bunter e. m. f. affects the operation of a series 
motor. ^ -^''^ 

4. What is meant by the emergency brake when applied to a series 
railway motor equipment ? 

5. How would you determine the counter e. m. f. of a series motor 
when operating at a certain current input under a given potential ? 

6. Draw a diagram showing how a series motor may be set up with 
starting resistance and also show how its direction of rotation may be 
changed. 

7. What do the characteristic curves of a series motor show, and 
what relation exists between torque and current input ? 

8. What method is used in a railway equipment to produce a con- 
stant rate of acceleration until almost up to speed ? 

9. Why is it necessary to have a series motor rigidly connected to 
its load ? 

10. About what relation exists between the armature and the field 
resistance of a series motor ? 



CHAPTER XI 



THE ARC LIGHT 



The Carbon Arc. — The electric arc was first produced by 
Sir Humphry Davy in 1805. The arc was formed between 
two wood charcoal pencils from a battery of 2000 cells. 
The name arc has been given to the bridge of conducting 
vapors because of its arched shape. To form an arc be- 
tween two electric light carbons it is first necessary to bring 
the carbons into contact and then to separate them a small 
distance, about \ inch. In the modern arc lamp means 
are provided to bring the carbons into contact, to separate 
them, and to feed the carbons together as they are con- 
sumed. These arc lamp mechanisms are automatic in their 
action. With the carbon arc almost all of the illumination 
is given off from the tips of the car- 
bons, which become molten at a tem- 
perature of about 4500° C. With the 
direct current carbon arc lamp, almost 
all of the illumination is given off 
from the upper positive carbon, the 
luminous part being termed the crater. 
With the direct current arc lamp, the 
positive carbon is consumed about 
twice as fast as the negative carbon. 
With an alternating current arc. Fig. 256, both carbons are 
equally luminous, and both are consumed at about the same 
rate. 

Practically no illumination is given off by the arc flame 
in a carbon arc. The distribution of candle power from 
an alternating current arc is about as shown in Fig. 256. 

1 86 




Fig. 256. — Alternating 
Current Arc. 



THE ARC LIGHT 1 8/ 

Experiment 115. Form an arc between two electric light carbons 
having a resistance in series with the carbons and a ii6-volt direct cur- 
rent source of supply. Preferably use an automatic arc lamp in a project- 
ing lantern. Project this arc on the screen in any one of a number of 
ways. Remove both condensers from the projecting lantern and bring 
the objective lens near the arc, focusing the arc upon the screen. Or use 
the condensers alone, inserting a card in which is a narrow slit ^-^ inch 
by I inch in the slide carrier ; then remove the objective lens, drawing the 
arc lamp back until a focus is formed. It may be necessary to remove one 
of the condensers. If this arc on the screen is observed in a darkened 
room, it will be noticed that the arc flame is practically non-luminous, 
having a central zone of bluish vapor, carbon monoxide. Outside of this 
zone there is a slight yellowish flame showing combustion taking place, 
the carbon monoxide burning in the air forming carbon dioxide. If an 
automatic lamp is used, the operation of striking the arc and feeding the 
carbons may be readily seen. As the carbons burn away, just before 
feeding the arc flame may travel around one of the carbons seeking the 
point of least resistance. Small bubbles of impurities will be noticed 
upon the positive carbon which will quickly dart across the arc flame. 

Experiment 1 1 6. Form an arc between a pair of carbons having an ad- 
justable resistance in series with the carbon and a i i6-volt direct current 
source of supply. Place an ammeter in series with the carbons and shunt 
the carbons with a voltmeter. Both ammeter and voltmeter may be pro- 
jecting instruments. When the lamp is operating, notice the voltage and 
current relations. The arc voltage should be about 40 volts if both 
cored carbons are used. Vary the distance between carbons and notice 
the change in potential values. At about 12 amperes, \ inch arc, the 
potential will be 40 volts and the resistance of the arc 3.33 ohms. 

Experiment 117. When the arc is projected on the screen, interpose 
in its path red, yellow, and blue glasses, and notice in each case that the 
arc assumes that particular color. 
This experiment shows that the 
temperature of the crater of the arc 
is so high that it emits light of all 
visible wave lengths. 

Experiment 118. Make a set- . . 

Fig. 21^7. — Method of obtaining 
up in a regular projectmg lantern. Spectrum. 

placing a card with a small slit in 
it in the slide carrier. Fig. 257. Interpose in the path of the rays 



CONDENSERS 
CARBONS 



A^ 



OBJECTIVE PRISM '^^ 



1 88 EXPERIMENTAL ELECTRICITY 

as they leave the objective lens a carbon bisulphide prism, and notice 
the pure spectj'iun on the screen. Compare this with a daylight 
spectrum. 

Physics of the Carbon Arc. — When the temperature of 
a body is raised, the body emits heat waves whose wave 
lengths are comparatively long compared with the wave 
lengths of light. As the temperature of the body is raised 
higher, it continues to emit these invisible radiations of heat 
and light, which are termed the infra part of the spectrum. 
The radiations vary approximately as the fourth power of 
the temperature. When the temperature of the body is 
about 1000° C. the body emits the first visible light rays of a 
dull red color. Scientists have shown that the red rays are 
not really the first light-giving or visible rays which actually 
appear, but that they are located in the yellowish green por- 
tion of the spectrum, the spectrum spreading out in both di- 
rections as the temperature is raised. The effect on the eye, 
however, is such that the impression of red is first received, 
the intensity of the other rays being too slight to affect the 
vision. After the red appears, the body whose temperature 
is being raised seems to be getting whiter and whiter. 
This is due to the fact that other wave lengths of light are 
appearing and interference is taking place, the colors which 
combine producing white light. As the temperature of 
the light source is raised, the efficiency of the illuminant is 
increased. This is probably due to the fact that more of 
the invisible radiations become visible at the higher tem- 
perature, the intrinsic brightness, or candle poiver per square 
inch of radiating surface, increasing. The temperature of 
the carbon arc has been given as approximately 4500° C. 
Although operating at this high temperature, the ordinary 
carbon arc has a very low Hght-giving efficiency — approxi- 
mately .85% for an open arc. It is claimed by mathemati- 



THE ARC LIGHT 1 89 

cians that by operating at a temperature of 7000° C. an effi- 
ciency of 50% could be attained. The efficiency of a light 
source may be determined from two factors, — the mechani- 
cal equivalent of light (.02 watts per candle power), and 
the wattage consumption of the illuminant under considera- 
tion. The mechanical equivalent of light is the wattage 
which would be consumed by an illuminant having an effi- 
ciency of 100%. 

The intrinsic brightness of some of our modern illumi- 
nants is given in the accompanying table : * 

Intrinsic Brightness 
C. p. per sq. in. 

Sun in Zenith .... 600,000 
Electric Arc . . . 10,000-100,000 
Calcium Light ..... 5,000 



Nernst Glower . 
Carbon Incandescent 
Gem Metalized . 
Tantalum . 
Tuno:sten . 



■^t)^ 



1,000 
480 
625 
750 

1,000 



Experiment 119. Take a small iron wire, No. 36, a few feet in length, 
and send through it a current of electricity of small magnitude, placing 
an adjustable resistance in series with the wire. Over the wire place a 
thermo element, as in Fig. 258, or, preferably, twist the wire several times 
around the element so that the rest of the wire will not get red hot and 
thus permit the element to conduct the heat away too rapidly. A copper- 
nickel thermo couple arranged with two junctions, one of which is placed 
in water, the other over the wire, leaving two free copper terminals to be 
connected to the galvanometer, affords a satisfactory arrangement. The 
fact that the two copper terminals are connected to the galvanometer 
eliminates temperature effects which would occur if only one junction 

* — From J. T. Wilse, in the Bulletin, for January, 1909, Brooklyn Section 
N.E. L.A. 

— From the Electrical Solicitor's Handbook, page 84, N, E. L. A. 




ESTON 



190 EXPERIMENTAL ELECTRICITY 

were used, especially where the galvanometer is placed before the pro- 
jecting lantern. The thermo element connected in the manner de- 
scribed will indicate the difference 

THERMO ELEMENT 

-• — -^ — ^ in temperature between the water 

and the wire. The Weston project- 
ing galvanometer described in the 
appendix, used frequently, will yield 
.085 scale deflection for 1° C. differ- 
'^aIvInometer ence in temperature, the resistance 

of the couple complete being 1.17 
Fig. 2c8. — Temperature shown with , \ ^ , ,. , 

^, ^, ^ ohms at 2 q C. An ordmary ffalva- 

Thermo Element. -' - *= 

nometer may be used by placing 
about 5000 ohms in series with it, and projecting an illuminated slit on 
the mirror, which will reflect the light spot on the screen. Send a cur- 
rent of electricity through the wire, and notice the gradual rise of tem- 
perature with the increase in luminosity. The wire will become a dull 
red at about 1000° C. Blow on the wire while it is a dull red, and notice 
how quickly it loses its luminosity due to the conducting air currents. 

Experiment 120. To show the illuminating power of some metals at 
lower temperature than the arc, introduce into a Bunsen flame salts of 
various substances, such as calcium, lithium, and strontium. 

Experiment 121. Over a Bunsen flame slowly drop a few iron filings 
and notice the brilliant shower of white sparks that result and that 
remind one of fireworks. This experiment shows that when the com- 
bustion of iron takes place, a pure white light, comparable with daylight, 
is produced. This principle is used in the magnetite arc lamp to be 
described later. 

Experiment 122. Blow through a small glass tube some lycopodium 
powder into a Bunsen flame, and note the brilliant illumination that will 
be produced. 

Experiment 123. Interpose sheets of iron, zinc, copper, etc., between 
the carbons of an arc lamp, and notice the additional illumination as 
combustion occurs. This is the principle of the flaming arc. 

So far, the infra rays and the rays of the visible spec- 
trum have been mentioned. Beyond the visible spectrum 
of red, orange, yellow, green, blue, indigo, and violet lie 
waves of light of still shorter wave length, termed the ultra 



THE ARC LIGHT 191 

rays. These rays excite fluorescence and penetrate animal 
tissue as does the sHghtly different X-ray. 

Efficiency of Light-giving Bodies. — It has been shown 
that as the temperature of a body is raised, its efficiency as 
a Hght producer increases. If, however, we can use sub- 
stances, such as calcium, which have a lower fusing point 
than carbon, an arc may be produced possessing a still higher 
efficiency. This is the principle of the flaming arc, the 
point being illustrated in Experiment 120. With the mag- 
netite arc developed by Professor Chas. P. Steinmetz, this 
feature is more largely utilized, for here is employed oxide 
of iron, FcgO^, a substance which has a much lower melt- 
ing point than carbon, and which yields illumination over 
the whole spectrum, thus producing the most efficient arc 
light we have. The flaming arc and the magnetite arc 
will be described in detail later. 

One of the most efficient of all illuminants to-day is the 
Cooper-Hewitt Tube, described on page 216. With this the 
light produced is not a function of the temperature, the 
illumination being probably set up by rapid oscillations or 
vibrations of the molecules of the gas. Heating the tube 
will not increase the efficiency of the light-giving source. 
The transformation of energy seems to be more direct, 
producing an efficiency of about .5 watts per candle power. 
This lamp is an x\merican product, the result of the in- 
vestigations of Peter Cooper Hewitt. In the development 
of the Nernst lamp, the osmium lamp, the tantalum lamp, 
and the tungsten lamp, efforts have been made to increase 
the efficiency by operating at higher temperature. The 
Geissler tubes used for many years experimentally in 
physics have very high efficiencies as light producers. 

The Ultra Rays. — For investigating ultra rays an induc- 
tion coil discharging between two iron electrodes, connected 




192 EXPERIMENTAL ELECTRICITY 

to its secondary winding and shunted with a series parallel 
arrangement of condensers as in Fig. 259, serves as an 

excellent producer of ultra rays. 
INDUCTION buch an induction coil can be oper- 

''°"' /\^VVVVV\ ated from a Wehnelt interrupter 

(see page 117) and from a direct 
current source of supply. 

Experiment 124. Operating such an 
CONDENSERS induction coil, hold near it a crystal of 
Fig 2-^0 —Iron Arc "" '"'^"^^ willemite, and notice that the willemite will 
fluoresce beautifully. Cover the willemite 
with a sheet of glass, and notice that the glass, opaque to the ultra 
rays, insulates them and that the willemite will not fluoresce. Inter- 
pose a sheet of quartz, and notice that the rays penetrate the quartz, 
causing the willemite to fluoresce. The rriercury vapor lamp is 
a great producer of ultra rays, and so is the sun, but with the 
mercury tube the glass retaining shell tends to insulate the passage 
of these rays. Mercury vapor tubes made from quartz are used abroad 
to sterilize water, the action of the ultra rays being destructive to animal 
tissue. Place near the discharge points of the induction coil a small 
glass bottle containing calcium sulphide, phosphorescent. Allow the 
rays to impinge upon the sulphide and then expose the tube to darkness. 
The bottle will phosphoresce. 

Experiment 125. Draw a large sign, such as EDISON, Fig. 260, 
on a chart and paint it with calcium sulphide, phosphorescent. Expose 
it to a Cooper-Hewitt tube and then expose it 
to darkness, and notice that the sign will phos- 
phoresce. 

Arc Lamp Circuits. — In order to 



EDISON 



. ^ . , . ^ ^ Fig. 260. — Sign, 

maintain a direct current open arc a 

potential of 40 volts is required across the arc terminals. 

The resistance of the arc varies with its length and cross 

section, the resistance of the carbons being quite small. 

In order to operate an arc it is necessary first to bring the 

carbons into contact and then to separate them. When 

the carbons are first in contact, the combined resistance of 



THE ARC LIGHT 1 93 

carbons and contact is small, as no arc exists ; it is so low, 
in fact, that if the carbons were placed across a potential 
of 40 volts a large current would pass. To regulate the 
starting current and also to help smooth out irregularities 
as the arc operates, a series resistance is inserted in the arc 
circuit. This resistance. Fig. 261, limits the 

- , , 116 VOLTS 

Starting current, and also consumes the excess v'-^-- 
potential when the arc is in operation. Assume 
an arc operating on a 11 /-volt circuit, the arc 
consuming 40 volts, and the series resistance 
consuming 77 volts. When the current passing 

through the arc would tend to fall as the arc re- I 

sistance increases, due to the carbons burning fig. 261.— 
away, the potential across the series resistance Simple Arc 
will decrease below 77 volts, and the arc voltage 
will increase above 40 volts. This series resistance is 
termed a balance coil, and it is a necessity in the arc circuit 
in order to obtain efficient operation. 

Various forms of mechanisms have been developed from 
time to time to feed together the carbons of an electric arc 
and to separate them when they are in contact. An exten- 
sive description of the more important of these 
mechanisms may be found in a small volume 
entitled Electric Arc Lamps, by Zeidler and 
Lustgarten. As most of the recent develop- 
ments in arc lamps have been foreign, this 
manual, which provides a resume of foreign 
practice, will be found helpful. The simplest 
mechanism, however, in extensive use in this 
country for both direct and alternating current 
arcs is shown in Fig. 262. It consists of a cir- 
cuit composed of a series resistance, a pair of magnets, 
and the carbons. These magnets draw up a plunger carry- 




194 EXPERIMENTAL ELECTRICITY 

ing a simple clutch. AVhen no current is passing through 
the lamp the clutch falls, allowing the carbons to pass 
through it as the clutch strikes a support, its circular 
opening becoming horizontal. This permits the upper car- 
bon to fall and strike the lower carbon. When the carbons 
are in contact and the circuit is closed, the series magnets 
draw up the clutch separating the carbons and causing the 
arc to be formed. As the carbons burn away, the current 
passing through the arc decreases, the strength of the 
series magnets decreases, and the armatures of the mag- 
nets fall slowly. When they have fallen a certain amount, 
the clutch supported by the armature strikes a support, 
causing the carbons to feed. In order that the readjust- 
ment of the carbons may not be too rapid, a dashpot is con- 
nected with the mechanism. This dashpot has a graphite 
plunger closely fitted to a cylinder. The dashpot is pro- 
vided with a needle valve, so that the carbons may be fed 
together quickly but may be separated slowly. An inter- 
esting device was used for many years in a number of arc 
lamps, notably the Continental Lamp employed by the Boston 
Edison Company. This device consisted of a bundle of 
soft iron wires placed in the top of the lamp, a few turns 
of wire in series with the lamp being passed around them. 
This device was in reality a choke coil and tended to resist 
the heavy starting current, the choke coil being effective 
when the current was rising from zero to its maximum 
value or when the current changed during operation. The 
device smoothed out many of the irregularities incident to 
operation, although it was rather heavy and cumbersome. 
Experiment 126. Place an ammeter in series with a direct current 
arc, and notice the starting current ; notice also the variation of current 
during operation. Measure resistance of carbons and series resistance 
with a voltmeter Avhen the arc is in circuit and calculate starting current 
on the assumption that no series resistance was present. 



THE ARC LIGHT 1 95 

Alternating Current Arc Circuit. — The alternating cur- 
rent arc mechanism is quite similar to the direct current 
mechanism, except that the armatures of the solenoid are 
supported by heavier springs on account of the vibration 
of an alternating current arc mechanism (G. E. Co.), and 
that, further, a reactance coil is substituted for the series 
resistance. This form of device is known as a series 
mechanism ; but there are two other types in use known as 
shunt and differential mechanism, each of which may be 
used on both direct and alternating current arc lamps. 
The shunt mechanism has a high resistance coil shunted 
across the arc terminal actuating the feeding mechanisms 
when the potential across the arc terminals rises beyond a 
predetermined point. A series resistance is also used with 
this mechanism. With the differential mechanism two coils 
work in opposition to each other, a shunt and series mag- 
net. Diagrams of the circuits of any of the various arc 
mechanisms may be readily obtained from the various 
manufacturing companies. 

Inclosed Arcs. — Owing to the fact that the carbons of 
an open arc have to be removed and new ones inserted 
after burning about ten hours, means were taken in the 
development of the inclosed arc to extend the life of the 
carbons. By inclosing the carbons in a small globe having 
a slight inlet for air, the arc was operated under a partial 
vacuum; thus the combustion and the current passing 
through the arc were reduced, and the pressure across the 
arc terminals was increased from 40 volts to 80 volts. It 
is very important to regulate carefully the exact amount of 
air which is allowed to enter the inclosing globe. If too 
much air enters the globe, the combustion of the carbons 
becomes too rapid, whereas, if the air supply is too small, 
not sufficient air will enter to consume the free carbon dust 



196 EXPERIMENTAL ELECTRICITY 

which comes off from the arc, and it will deposit in a fine 
powder over the surface of the retaining globe. This ad- 
justment is arranged very ingeniously in the G. E. Co.'s in- 
closed arcs by a spiral path which the air has to take through 
the cap of the globe before it enters the globe proper. In- 
closed arcs are used extensively for street lighting and for 
interior lighting, but they are being rapidly superseded in 
all classes of work by the more efficient tungsten lamp. 

Recent Development in Arc Lamps. — In addition to the 

flaming arc and the magnetite arc, previously referred to, 

several minor developments in arc lamps may be mentioned. 

Although a few improvements have been made in this 

country, most have been made abroad. The Daylight lamp 

is a semi-inclosed arc which uses small carbons, 6 m.m. 

diameter, and burns at 5 amperes at 80 volts, operating 

semi-inclosed. It contains no inner globe, but the outer 

globe is of a mushroom shape fitted with a ground edge 

which is held against a flat metal surface by a spring. The 

small carbons prevent the wandering of the arc, and the 

shape of the opal globe assists diffusion. The life of a 

trim is about 30 hours, the remains of the top carbon being 

used for the bottom trim. In the Daylight lamp, as the 

air becomes heated, it is driven out of the inclosing globe. 

Sometimes these globes crack from sud- 

^ den pressure. An improvement with the 

M M flaming arc has been to provide a cup- 

M V shaped inclosing device of fired clay in 

M U which the carbons form their arc. This 

M V tends to increase the life of the carbons. 

^^^ Both carbons also feed downward, as in 

FIG. 263 -Both Car- -p- ^63, the arc being formed across 

bons Down-feed. fc. J> & 

their tips, producing a distribution which 
is superior to the ordinary vertical arc where the positive 



THE ARC LIGHT 1 97 

carbon is above and the negative carbon is below, casting 
a heavy shadow under the lamp. 

Factors to be considered in selecting an Illuminant. — At 

present there is a tendency in selecting an illuminant to be 
governed entirely by its efficiency. Other factors, such as 
first cost, color of illumination, distribution, maintenance, 
and intrinsic brightness of illuminant should be considered. 
The Cooper-Hewitt tube, for instance, is one of the most effi- 
cient of illuminants expressed in watts per candle power, it 
has a uniform distribution, great detail-revealing powers, and 
a life practically infinite, but it is limited in its application by 
its color. This characteristic, however, is being overcome 
by operating the lamp in the same inclosing outer globe 
with a tungsten lamp, thus producing a combined efficiency 
not quite so high as before, but yielding a light better 
adapted for general illumination. Owing to the absence 
of red rays, or that part of the spectrum where the radiant 
heat rays are present, the fight is what may be termed a cold 
light, and is, therefore, very restful for the eyes to work under. 
The high efficiency of the light is probably caused by the 
fact that its illumination seems to be concentrated around 
the yellowish green part of the spectrum, or at that point 
where the luminosity is the greatest. This light also has 
a low intrinsic brightness compared with other illuminants. 
In the tungsten incandescent lamp the efficiency and life 
are much greater than the carbon lamp, the light is much 
whiter, as the lamp operates at a higher temperature than 
the carbon incandescent lamp, but the cost is greater and 
the filament more fragile. In the Moore tube the distribu- 
tion is as nearly like daylight as can be produced, especially 
when the tube is filled with carbon dioxide. The color 
resembles daylight, and the intrinsic brightness of the tube 
is about the lowest of any known bare illuminant. The 



198 



EXPERIMENTAL ELECTRICITY 



efficiency of this apparatus, however, is not quite so high 
as that of the Cooper-Hewitt tube. As regards the distri- 
bution of arc lamps, the alternating current arc has the 
power of reaching out over a greater distance with its Hght 
than a direct current arc, and it requires a smaller number 
of arcs to cover the same territory, owing to the fact that 
both carbons are luminous. But under some conditions the 
operation of such a system may not be so efficient as the 

direct current system. The use 
of a Holophane reflector. Fig. 
264, with a tungsten lamp with 
a frosted tip forms one of the 
most beautiful and efficient illu- 
minants we have. The intrinsic 
brightness of the combined fix- 
ture is low, the efficiency is high, 
and the distribution may be 
made almost anything desired 
according to the shape of the 
reflector. Care must be taken 
in the use of illuminants not to 
have the bare lamps directly in 
the field of vision, as their high intrinsic brightness tends 
to injure the eye, owing to the limits of range of contraction 
of the pupil, and also to interfere with one's ability to read ; 
thus, a i6-candle-power lamp in the field of vision de- 
creased one's ability to read by about 30%. With the 
Nernst lamp the distribution of the light is quite uniform, 
the gradual illumination of the lamp is pleasing, its intrinsic 
brightness is fairly low when equipped with a frosted dif- 
fusing globe, and its efficiency about equals that of the 
tantalum lamp. 

The Flaming Arc. — The flaming arc consists of carbons 




Fig. 264. — Holophane Reflector. 



THE ARC LIGHT 199 

with their cores impregnated with calcium and strontium, 
having lower points of combustion than the carbon sheel. 
These substances upon combustion have a high illumi- 
nating value, the consumption of the arcs being .3 watts 
per candle. Owing to the fact that the illumination is given 
off largely by the arc flame, any irregularities in the opera- 
tion of the arc cause flickering. The lamps are useful for 
street illumination where a large volume of light flux is 
required. 

Experiment 127. Throw on the screen by means of a projecting 
lantern (see page 187) a flaming arc. Notice the continuous shifting of 
the flame, and notice the illumination of the flame. Introduce carbons 
impregnated with various substances giving a yellow arc, a white arc, 
and a golden arc. 

Experiment 128. Throw the spectrum of this arc on the screen by 
arranging the lantern for ordinary projection, inserting a small slit in 
the slide carrier, and placing a carbon bisulphide prism before the pro- 
jecting lens. Notice the broken spectrum lines, and the predominance 
of green and yellow bands. The green band is a characteristic of most 
metals. 

Experiment 129. When using a carbon arc and a prism, it is inter- 
esting to show that the ordinary green, blue, and yellow glass employed 
commercially is not monochromatic. Place these glasses, one at a time, 
before the prism, and notice that other colors pass besides the color 
indicated visually. A green glass will pass yellow and blue rays and 
sometimes a little red. The red glass is fairly monochromatic, however. 

The Magnetite Arc. — The magnetite arc was developed 
by Charles P. Steinmetz of the General Electric 
Company. The purpose of using magnetite, one of 
the oxides of iron, was to have a metal, which, 
when consumed, would give a pure white light. The 
spectrum of iron, as may be readily observed, is almost 
pure. In the magnetite lamp the positive electrode 
is made of a bar of copper in the form of a button, 
and the negative electrode is made of an iron tube filled 



200 EXPERIMENTAL ELECTRICITY 

with magnetite. The oxide of iron to be used must be 
such that it will not disintegrate at the ordinary tempera- 
tures. With the carbon arc the illumination is given off by 
the carbon tips heated to incandescence, whereas with the 
magnetite arc the illumination is given off directly by the 
flame of the lamp. It may be noted, therefore, that with 
the magnetite arc, the distribution of light will be more uni- 
form, and there will be fewer shadows, than with the carbon 
arc. One difficulty, however, results from the flame illumi- 
nation, for owing to the flickering of the arc the illumination 
is somewhat unsteady. Because of the consumption of the 
iron electrode by the arc, fumes are generated that result 
in a heavy brick-red deposit. This deposit is being 
minimized in the modern lamps by special draft tubes. 

QUESTIONS 

1. Explain in detail the operation of an arc light, drawing a diagram 
of circuits, lamp employing ordinary carbon electrodes, series mechan- 
ism for multiple circuit. 

2. Explain the diiference between an ordinary carbon arc, a flaming 
arc, and a magnetic arc. 

3. Give the color values, distribution of light, and watts per candle 
power of open carbon arc, flaming arc, and magnetite arc. 

4. How do the temperature values of the carbon arc, the flaming 
arc, and the magnetite arc compare? 

5. What is the function of the series resistance in arc lamps ? 
Why would it not be better to eliminate this series resistance from the 
circuit ? 

6. Give an experiment to show that the arc flame is a conducting 
medium. 

7. What is a cored carbon? How will it help to steady the arc? 

8. What is the difference between a differential and a shunt mech- 
anism? 



THE ARC LIGHT 20I 

9. Mention a few of the modern German improvements in arc lamp 
manufacture. 

10. In using an alternating current arc, why is it desirable to use a 
reflector? How do the luminosity, rate of feed, etc., of direct current 
and alternating current arcs compare ? 

11. Why is an open carbon arc more efficient in watts per candle 
power than a carbon incandescent lamp? 




CHAPTER XII 
INCANDESCENT ILLUMINANTS 

The Carbon Incandescent Lamp. — To Thomas A. Edison 
is undoubtedly due the credit of inventing and placing in 
practice in the United States the first success- 
ful commercial incandescent lamp. The Edi- 
son lamp was patented in 1878.. Other men — 
J. W. Starr of Cincinnati, in 1845, and J. W. 
Swan of England, in 1878 — had invented in- 
plp llpli candescent lamps before Edison's patent was 
^ifeMiyy issued, but Edison realized that in order to 
make a success of the incandescent lamp it was 
Carbon In- ncccssary to perfect generating and distributing 
candescent systems as Well. Accordingly, he developed 
Lamp. ^]^g Edison Generator, the Edison Three-wire 
System of Distribution, the Edison Underground Tube 
System of Distribution, and many other elements of the 
first fighting systems with which we are all acquainted. In 
the early days of the Edison lamp, men were sent all over 
the world in search of a suitable material for a filament. A 
special kind of bamboo was finally selected. The principle 
of the incandescent lamp as developed by Edison was to 
inclose a carbonized filament in a vacuum tube and to ex- 
tract all of the air so that combustion of the filament could 
not take place when the filament had been rendered incan- 
descent by the passage of an electric current. In the 
early experiments with the bamboo filament, the lamps had 



INCANDESCENT JLLUMINANTS 203 

an efficiency of about 5.8 watts per candle power. Later, 
when the lamps were finally issued for commercial use, this 
was reduced to 4.6 watts per candle power. 

Experiment 130. Partially submerge in water an incandescent 
lamp in a vertical position, butt end up and tip end down. With a 
pair of pliers break the tip of the lamp off under water, and notice the 
rush of water up into the lamp due to atmospheric pressure outside and 
the presence of a vacuum inside. 

Edison's early filament of carbonized bamboo has been 
superseded, as the early filaments were not uniform. The 
modern filaments, Fig. 269, are uniform, cheaper, have a 
longer life, and are more economical in every way. In 
making the modern filament absorbent cotton is dissolved 
in chloride of zinc and hydrochloric acid, and stirred until 
it forms a gelatinous mass like heavy molasses. This ma- 
terial is placed in glass bottles, as in Fig. 266, with a small 
opening in the bottom which empties into a vessel contain- 
ing an upright cylinder and alcohol. A slight air pressure 
is applied to the top bottle, squirting the gelatinous mass 
through the opening into the lower vessel in a hairlike 
thread resembling fine spaghetti. Care must be taken 
throughout the process to allow for shrinkage and to keep 
the shrinkage uniform. The lower bottle turns as the 
process continues, the raw filament winding itself upon the 
upright support. The process is called squirtijig. Later 
this drum is removed and in the drying room the fila- 
ment is washed and wound upon a drum (four feet in 
diameter). Fig. 267, and allowed to dry. A knife blade is 
then passed along parallel to the axis of the drum, cutting 
the filaments in lengths of about 14 feet. These filaments 
are sorted roughly for size ; they have the shape of coils 
about 6 inches in diameter and are placed in drying closets. 
When dry they are placed upon frames to give the desired 



204 



EXPERIMENTAL ELECTRICITY 




INCANDESCENT ILLUMINANTS 



205 




206 EXPERIMENTAL ELECTRICITY 

shape — -oval coil, double coil, spiral, or hairpin — and are 
placed in drying ovens, where they are left for two hours, 
the temperature being 450° F. They are then taken out 
and cut into separate filaments. In bunches of fifty these 
separate filaments are dipped in paraffine, making one large 
filament \ inch by y^g inch in diameter. 

They are now ready to be carbonised. This process 
takes about 8 days. The filaments are packed in iron 
boxes, surrounded with peat, and placed in air-tight fur- 
naces, the temperature being raised to 1800°. After a 
week the furnaces are allowed to cool, and their contents 
are placed in air-tight graphite crucibles and cemented. 
The crucibles are next placed in another furnace at 3200° 
F. for two hours. A metaUized filament is allowed to re- 
main in the furnace an additional hour. The filaments are 
then allowed to cool gradually. An extra coating of car- 
bon is next deposited upon the filament to make it of uni- 
form resistance. This is termed treating. In treating the 
filament each operator has four bottles, a relay, and an 
ammeter. These bottles have their tops flush with the 
table and are three inches in diameter; each has a two-inch 
mouth fitted with a soft rubber bushing to make it air-tight 
when the cap is closed. The filament is supported in the 
cap with two insulated clamps to which leads are con- 
nected, the leads coming out through the cap terminals in 
two small clips. These clips make contact with two simi- 
lar clips on the table when the cap is placed in position 
on the bottle. The operator inserts a filament in the 
clamps, and places the cap in position, causing the fila- 
ment to extend down into the bottle. A small port is then 
opened in the bottom of the bottle, and the air is exhausted 
to nearly a perfect vacuum — -^-^ oi i %. The port is then 
closed, and another port is opened, admitting gasoline 



INCANDESCENT ILLUMINANTS 



207 



vapor. As soon as the bottle is filled, the port is closed 
by a cam which also sends current through the filament. 
A voltage about 75 % above that upon which the filament 
is intended to operate is used. When the current passes 
through the filament, it decomposes the gasoline, depositing 
carbon on the filament. 

The deposit makes a uniform filament, for if one part of 
the filament is of smaller cross section than another, its re- 
sistance is greater and it reaches a 
higher temperature than the other 
parts, decomposing more of the vapor 
and depositing more carbon at that 
particular point. When the resistance 
of the filament reaches a predeter- 
mined value, the relay automatically 
cuts off the current. The exhaust 
port is then opened, allowing the 
burned gases to escape, destroying 
the vacuum, so that the cap may be 
easily removed. The whole process 
of treating a filament takes about 1 5 
seconds. As the operator has the 
four bottles working at different stages 
of the process, the output is rapid. 

The glass bulbs as they come from 
the manufacturer have a round face to 
which is attached a short neck in the 
factory, forming what is termed a tabu- 
lated bulb, Fig. 268. They also have 
a mouth about 3 inches long, as in Fig. 
268. The carbon filament is cemented 
at its ends to two small pieces of _ 

^ Fig. 268. — Bulb with Tip 

platmum, which have been previously Added. 




208 



EXPERIMENTAL ELECTRICITY 




Fig. 269. — Fila. 
ment Mount. 



sealed in a glass stem, Fig. 269. To the other end of the 
platinum wires are soldered two copper lead- 
ing-in wires. The platinum wires are neces- 
sary where the wires pass through the glass, 
as platinum has the same coefficient of expan- 
sion as the glass, and the glass accordingly 
will not break. The glass butt is then sealed 
in the end of the bulb, both being sealed to- 
gether by heating their point of contact. Fig. 
270. The filament is then 
connected through the 
leading-in wires to a source 
of current supply, and the 
end of the stem of the glass 
bulb is connected to an air 
pump. Fig. 270, and a par- 
tial vacuum is produced. The operator 
is able to tell from the color of the lumi- 
nous conducting gas in the lamp when 
the vacuum is nearly perfect. When 
this point is reached, a small amount of 
red phosphorus previously inserted in 
the stem of the tube is heated, forming 
a gas which fills the space of the rarefied 
air in the bulb. The stem is then 
sealed off, forming the well-known tip 
on the lamp. The lamp is then sub- 
jected to a series of tests ; its vacuum 
is tested by an induction coil, its fila- 
ment is tested for bright spots, and the 
voltage for which the lamp will give the 
designed candle power is determined 




Fig. 270. — Bulb ready 
for Exhaust. 



on 



a photometer. 



This voltage is then indicated on the label of the lamp. 



INCANDESCENT ILLUMINANTS 209 

In this description some of the smaller steps have been 
eliminated, owing to limited space ; but further details will 
be found in a pamphlet on Incandescent Lamps issued by 
the Sawyer- Man Electric Company, now controlled by the 
Westinghouse Electric Manufacturing- Company. This 
pamphlet is designated S. M. Form No. 721. The sub- 
ject is also discussed in a paper presented by E. B. Ran- 
nells before the Brooklyn section of the National Electric 
Light Association, Vol. I, December, 1908, page 29. 

Characteristics of Filaments. — The termination of the 
useful life of a filament, or what is termed the smashing 
point, is reached when the filament falls in 
candle power to 80 % of its original value. 
The average useful life of a carbon filament 
lamp is about 600 hours. The carbon filament 
lamp possesses a negative te^inpeTatiLve coefficient ; 
that is, it has a higher resistance when cold than 
when hot, the resistance of a i6-candle-power 
carbon filament lamp of the Edison type being fig. 271.— 
about 500 ohms when cold and 250 ohms when Tantalum 
hot. This produces a lamp whose illumination e.s.Co.).' 
falls off rapidly with a decrease in voltage, a 
ii6-volt lamp yielding zero illumination at about 30 volts. 
The average wattage consumption of a carbon filament 16- 
candle-power lamp is 3.1 watts per candle power. By 
treating the filament to a higher temperature, or carbon- 
izing it, the metallized filament, or Gem lamp, is produced, 
having a wattage consumption of 2.5 watts per candle 
power. Among the latest types of illuminants are the 
tantalum lamp, Figs. 271, 272, having an energy consump- 
tion of 2.25 watts per candle power, and the tungsten lamp, 
Fig. 275, of I to 1.25 watts per candle power. Since both 
the tantalum lamp and the tungsten lamp filaments are 




210 



EXPERIMENTAL ELECTRICITY 



made of metal, they have positive temperature coefficients. 
Lamps having positive temperature coefficients have their 
lowest resistance when cold. Under 
these circumstances a lamp such as 
a tungsten lamp will become lumi- 
nous at a much lower voltage than 
a carbon lamp, the filament being 
visible at lO volts. This character- 
istic is especially important, for if 
the pressure on the system should 
become low, due to increase in load, 
a tungsten lamp will yield a much 
higher illumination on the reduced 
voltage than a carbon lamp of the 
same candle-power unit. At 80 
volts, for instance, a ii6-volt tung- 
sten lamp will yield considerable 
illumination, whereas a carbon lamp 
w^ould be reduced to a very low 
candle-power value. 

Experiment 131. Arrange the circuit as in Fig. 273, placing ammeter 
shunt in series with a lamp socket, a ii6-volt source of potential, and a 
high resistance which can be in- 
serted by means of a fuse plug. 
Wire projecting galvanometer to the 
middle terminals of a double throw 
switch, and connect one side of 
the switch to the ammeter shunt, the 
other side being connected to the 
two terminals of the lamp through 
the voltmeter resistance. When 
the switch is thrown in one direc- 
tion, the galvanometer will indicate 
amperes; when it is thrown in the 

opposite direction, it will indicate volts. Measure the resistance of car- 
bon, tantalum, and tungsten lamps when illuminated so that they are 




Fig. 272. — Tantalum Fila- 
ment. 




O )l-AMP SOCKET 



V\AMAr 



voltmeter resistance 
Fig. 273. — Circuit illustrating Posi- 
tive and Negative Temperature 
Coefificient. 



b 




^ 



INCANDESCENT ILL UM IN ANTS 2 1 1 

hardly visible, and then remove external resistance, insert the fuse plug, 
and measure the resistance of the lamps hot. Show that lamps change 
their resistance and that the resistance of the carbon lamp decreases 
with an increase in temperature, whereas the resistance of the tantalum 
and the tungsten lamps increase with increase in temperature. 

Experiment 132. Connect a tantalum, a tungsten, a metallized car- 
bon, and a carbon lamp in parallel, and place the combination in series 
with a high adjustable resistance and a ii6-volt source of potential. 
Shunt the lamp board with a projecting voltmeter so that ^ ^^^^^ 
voltage across the lamp terminals for each adjustment of "j" '^•''• 
the series resistance will be indicated. Start with about 
5 volts and gradually raise the pressure until it is about 
150 volts, using ii6-volt lamps. When the voltage is 10 
volts, the tungsten filament will be visible; when the 
voltage is about 15 volts, the tantalum filament will be 
visible ; when the voltage is about 25 volts, the metallized 
filament will be visible ; and when the voltage is 35 volts, 
the carbon lamp will be visible. Continue the raising of 
the voltage, noticing that below 116 volts the carbon fig. 274.— Cir- 
filament lamp increases in candle power per unit of voltage cuit to show 
change less than the other lamps. Above 116 volts the Variation in 
carbon filament lamp will increase in candle power more Candle Pow- 

rapidly than the others. This is due to the fact that the ^^ ^'^^^^ ^^^^" 

sure, 
resistance of the other lamps above 116 volts increases 

with an increase in temperature, whereas the resistance of the carbon 

filament decreases. 

The Tungsten Lamp. — The tungsten lamp, as has been 
said, is one of the latest and most efficient incandescent 
lamps now in use. The tungsten used in making the fila- 
ment of this lamp is obtained from the mineral wolfram. 
Wolfram is a rare mineral commonly known as tungstate 
of iron and manganese. As a metal tungsten naturally 
has a low resistance, which makes it necessary that the 
filament be long and fine in order to have sufficient resist- 
ance to limit the current input when connected across 
a ii6-volt circuit. Lamps of the higher candle powers 
require less resistance and can therefore be made more 



212 EXPERIMENTAL ELECTRICITY 

substantial than the small-sized units. Small candle-power 
units such as i6 candle power are not much used on the 
regular ii6-volt system. They are being employed, how- 
ever, for series sign lighting in which the lamps can be 
made for lower voltage operation and connected in series. 

The Tungsten Filament. — The tungsten filament is made 
from tungstic oxide, which is obtained in an impure state 
as a by-product from mines in Colorado. It is mixed with 
iron, nickel, arsenic, etc., and is refined into pure tungstic 
oxide, in which state it is of a fine yellow color in the form 
of a powder resembling somewhat powdered sulphur. The 
process of reduction results in the pure tungsten metal, 
which is a black powder. This tungsten powder is mixed 
with molasses and evaporated till nearly dry; then it is 
rolled between steel rollers until it resembles pure rubber. 
The tungsten mixture is next formed into a thread by plac- 
ing small pieces of it 2 inches by \ inches in size into a 
steel cylinder, at one end of which is placed a die cut from 
a diamond in which is a small hole varying from 2 to 50 
mills in diameter. A piston is then fitted in the other end 
of the cylinder and this forces the tungsten through the 
hole in the die and produces a fine thread. The thread is 
caught on a white card which the operator passes back 
and forth under the die in both directions, moving it far 
enough to produce a loop double that required for the fila- 
ment. When the cardboard is filled, a knife is passed 
over the loops at the center, leaving a number of single 
loops. These filaments are packed in peat dust, baked in 
an atmosphere of inert gas, and treated and flashed in an 
atmosphere of the same gas. 

The tungsten filaments are so fine and delicate that the 
operators have to handle them with long needles, and even 
with this care the great majority of them are broken before 




INCANDESCENT ILLUMINANTS 213 

they are mounted in the globes. This filament, owing to 
its fragility, is mounted in a somewhat different manner 
from the carbon filament. In Fig. 275 it will be 
noted that the filament is supported on a glass 
pedestal. This pedestal consists of a glass rod 
\ inch in diameter fastened midway between 
the leading-in wires. Upon this pedestal are 
mounted two collars carrying small tantalum 
holders which support the tungsten filament. 
The entire length of the filament is about 2 feet. 
An objection against the tungsten lamp as fig. 275. 
first introduced was that it had to be sus- Lamp"TM^^E 
pended in a vertical plane because the tung- s. Co.). 
sten at incandescence becomes quite soft. This defect 
has been remedied to a great extent through the introduc- 
tion by the General Electric Company of the 25-wattlamp, 
in which the loops of the filament are supported at their 
middle so that the lamps can be placed at any angle. 
The tungsten lamp bums just as well on alternating as on 
direct current, an advantage not possessed by the tantalum 
lamp. 

Owing to the small diameter of the tungsten filament it 
requires greater care in handling and suffers more from 
shocks ; at present its cost is several times larger than 
that of the carbon lamps, but its great gain in energy 
consumption and its long life of 2000 lamp hours and its 
white hght make it superior and more economical. Owing 
to its high intrinsic brightness (see page 189), this lamp 
should never be exposed directly to the field of vision. 
A Holophane reflector employing an 80-watt tungsten 
lamp with a frosted end gives a low intrinsic brightness, a 
beautiful illuminating effect, and an excellent distribution 
of light. Two such lamps properly located will light a 



214 



EXPERIMENTAL ELECTRICITY 



small store advantageously, and one such fixture will light 
a good-sized room in a private residence. Another desir- 
able characteristic of the tungsten lamp is the whiteness of 
its light, which is due to the high temperature of its fila- 
ment. Tantalum and tungsten have lower melting points 
than carbon, but they have higher evaporatiojt points and 
so can be operated at higher temperatures. 

The Moore Vacuum Tube. — Physiologically the most 
satisfactory illuminant is that which will cause the least 
harm to the eyes. This condition' is fulfilled more by the 

Moore tube, Fig. 276, 
than by any other elec- 
trical illuminant, its in- 
5^E'msTmBUTED,M trinsic brightness, 

ANY FORM DESIRED J 1 i. j. 

TO LENGTHS OF 200 Fj. Candle powevpev square 




Fig. 276. — Moore Tube. 



inch of dijfiising sur- 
face, being 0.66 candle 
power per square inch, 
corresponding to 12 
candle power per linear 
foot of tube. The 
Moore tube gives out 
a soft diffused light 
closely resembling daylight when the tube is filled with nitro- 
gen, and having a light red, orange, or salmon color. This 
system was invented by O. McFarlan Moore and was intro- 
duced commercially in 1903. The illuminant, as in Fig. 276, 
consists of a glass tube of any length up to 200 feet and i| 
inch in diameter of any desired form. The tube may be 
made up in the form of letters as for a sign, or it may 
extend around the ceiling of a room, being supported at 
intervals of 8 feet. The apparatus consists of a static 
transformer connected to two graphite electrodes sealed in 



INCANDESCENT ILLUMINANTS 



215 




the ends of the tube, which is filled with air, nitrogen, car- 
bon dioxide, or any other suitable gas. The tube contain- 
ing the gas is then exhausted to a pressure of about -^-^ mm. 
of mercury. As the tube operates like 
all vacuum tubes, the vacuum becomes 
more and more perfect, altering the 
efficiency of the light, the most efficient 
point of which is about o.i-o. 12 mm. 
The conductivity of the gas, however, is 
a maximum at 0.08 mm. of mercury, the 
current consumption at this critical point 
being a maximum. To regulate the vac- 
uum a separate tube projects downward 
from the main tube to what is termed 
the feeder valve. Fig. 277. The feeder 
valve consists of a solenoid connected in fig. 277.— Feeder Vaive 
series with the primary of the trans- for Moore Tube. 
former. This coil controls an iron plunger, raising or low- 
ering it. The iron plunger is fastened in a glass displac- 
ing tube. 

In the end of the by-pass tube is cemented a carbon plug 
with a small opening in it which is normally covered with 
mercury. As the displacing tube is moved up, the mercury 
recedes, uncovering the opening in the plug and allowing 
a small amount of air to enter the tube. As the vacuum 
in the tube increases, its resistance decreases, and conse- 
quently the current passing through the tube increases. 
An increase in the secondary current is accompanied with 
an increase in the primary current in the transformer (see 
the chapters in text on Transformers). The increased 
primary current increases the current passing through the 
series coil on the feeder valve, raising the plunger as pre- 
viously described. When the vacuum is restored to normal 



2l6 



EXPERIMENTAL ELECTRICITY 



value by the admission of air, the current input in the 
primary decreases, the plunger in the feeder valve falls, 
and the mercury covers the opening in the plug, excluding 
a further supply of air. The transformer is usually oper- 
ated at a primary voltage of 220 and a secondary voltage 
of 2200 volts. The secondary voltage varies with the 
length of the tube. The candle power of the tube can 
be varied from o to 50 candle power, giving normally about 
12 candle power per foot of tube. The efficiency of the 
tube varies from 1.5 to 2 watts per candle power, varying 
with the length of the tube. The tube when arranged 
around the corners of a room is most artistic, producing a 
mellow light upon which the eye can gaze from time to 
time without apparent fatigue. 

The Cooper-Hewitt Mercury Vapor Lamp. — The Cooper- 
Hewitt lamp, Figs. 278, 279, consists of a glass vacuum 

tube having a mercury 
kathode in one end of 
the tube and an iron 
anode in the other end. 
Electric current is fed 
into the lamp through a 
suitable ballast resist- 
ance to two platinum 
wires passing through 
the tube to the two elec- 
trodes. The lamp oper- 
ates on 115 volts direct 
current and takes about 
6 amperes. At starting 
FIG. 278. -Cooper-Hewitt Tube. ^^^ j^.^j^ resistance of 

the negative electrode must be overcome. Several methods 
have been introduced for accompHshing this, such as using 



U8ETHEBE POSTS FOil 
110-120 VOLTS 

loo-<no « 




INCANDESCEIVT ILLUMINANTS 



217 





--J 


















lamp] 


S 


^ 




•«• 


1 




^ 






^WWH 






^ 








^ QUICK-BBEAK SWITCH 


< RESISTANCE 














STARTING [... 


\ 








BAND 1F^ 






-O)000000fl]L< 1 




V 


) — 



INOUCTANC&.COIL 



Fig. 279. — Circuits of Cooper-Hewitt Tube. 



a momentary high potential from an induction coil; but the 
simplest and most satisfactory method is to simply tilt the 
tube, allowing a small stream of mercury to extend from 
one end of the 
tube to the other, 
the lamp lighting, 
allowing the tube 
to then return to 
normal position. 
In erecting the 
tube care must be 
taken not to screw 
the end clamps 
supporting the 
tube too tight, as the glass is necessarily thin, owing 
to the quick temperature changes it undergoes at starting. 
The lamp should in reahty be classified as an arc lamp 
instead of an incandescent lamp, as the illumination is 
given off by the incandescent mercury vapor, the illumina- 
tion probably resulting from some form of electro-lumi- 
nescence of the vapor. As a consequence of this condition, 
the efficiency of the lamp is very high, \ a watt per candle 
power, and is not affected by an increase in temperature. 
With proper care the life of the tube is practically infinite. 
The only objection urged against the tube is the fact that 
the absence of red rays in its illumination causes objects 
to appear in unnatural colors. Although this effect is 
objectionable so far as the personal appearance of individ- 
uals is concerned, it is highly important from a physiological 
viewpoint (see notes, page 197, in chapter on Arc Lights). 
A certain amount of induction and resistance is placed in 
series with the lamp, which seems to steady the arc. 



2l8 



EXPERIMENTAL ELECTRICITY 



Experiment 133. Take a spectrum chart and expose it first to a 
Cooper-Hewitt light and then to a tungsten lamp. Notice the change 
in color values. 

Experiment 134. Take two cloths, one a deep red, and the other 
a black, and expose them first to the Cooper-Hewitt tube and then 
to a tungsten lamp and note the absence of red rays in the Cooper- 
Hewitt lamp. 

Experiment 135. When the tube is lighted, hold a pointer about one 
foot distant from it, and notice the faint shadow cast upon the wall by 
the pointer. The distributing surface of the tube is so great that there 
is almost an entire absence of shadows. Repeat the experiment, using 
an arc lamp instead of the Cooper-Hewitt tube, and notice intense 
shadows. 

Experiment 136. Take a small bottle of calcium sulphide phospho- 
rescent and expose it to a Cooper-Hewitt tube. Turn off the light, 
and notice the phosphorescence of the bottle in a darkened room. 

The Nernst Lamp. — The Nernst lamp, Fig. 280, is the 
development of Professor Walther Nernst, of Gottingen 

University. Professor 

Nernst developed the 
Nernst lamp while inves- 
tigating certain substances 
such as thorium, cerium, 
zyrconium, erbium, ty- 
thrium, and glucinum used 
HEATER PORCE^ilNrQ jn thc devclopmcnt of the 

HEATER TUBE to ^ 

Welsbach mantle, among 
which was magnesium ox- 
ide mixed with porcelain. 
This is a high insulator 
when cold, but a good elec- 
trolytic conductor when 
hot. The present Nernst 




HOLDING SCREW 

' ALUMINUM PLUG 

1 ARMATURE SUPPORT 

L.POST 

BALLAST 

2~ CUT OUT COIL 

ARMATURE 

~-j:::;silver contact stop 

HOUSING 

contact sleeve porcelain 
— -globe holding screw 

HOLDER porcelain Jtj 



GLOWER ' I 



GLOBE 



Fig. 2S0. — Westinghouse Nernst Lamp. 



glowers. Fig. 281, as manufactured by the Westinghouse 
Electric Manufacturing Company, are made from kaolin. 



INCANDESCENT ILLUMINANTS 



219 




Fig. 281. — Glower. 



LAM-J3 rceikiiNAi.% 



Much difficulty has been encountered in the introduction 

of this lamp in America, owing to its fragile nature and the 

desire to make the lamp automatic in its 

operation. In foreign countries the lamp 

was introduced without the automatic 

starting device, the individual using the 

lamp heating up the glower with a 

match. Owing to the high resistance 

of the glower at starting, the lamp is pro- 
vided with a heater which is shunted 

with the glower, Fig. 281. This heater 

is automatically cut out of the circuit 

by an electro-magnet when the glower begins to conduct. 

A ballast resistance, Fig. 282, placed in series with the 

glower, having a positive 
temperature coefficient, 
neutralizes the decreased 
resistance of the glower 
when operating. The 
heater contains a coil of 
platinum wire. The 
color of the light is 
agreeable, being midway 
between the yellow and 
red rays of the incandes- 
cent lamp and the violet 
and blue of the arc Hght. 
The intrinsic brightness 
of the lamp is quite high, 
the glower necessarily 
being inclosed in a ground 




3 Glower Lamp 

Fig. 282. — Circuits for Nernst Lamp. 



glass globe. The life of the Nernst lamp glower is given 
by the Nernst Lamp Com.pany for the alternating current 



220 EXPERIMENTAL ELECTRICITY 

glower at 800 hours when operated at 60 cycles. There 
are two types of the glowers, depending upon whether 
they are intended for direct or alternating current operation. 
The ballast resistance has a life of over 20,000 hours, and 
the heater a life of about 8 months. The alternating cur- 
rent glower has received a greater development in America 
than the direct current glower, and consequently it has a 
longer life. The wattage consumption per candle power 
varies with the number of glowers, owing to the mutual 
heating effect which one glower has upon the other, 
the luminous radiation varying as the fourth power of 
the temperature (law of Stefan and Boltsman). For a 
6-glower lamp, clear globes, the wattage per hemispherical 
candle power is 1.88. For a 3-glower lamp the value is 
2.33. 

QUESTIONS 

1. What is the object of the vacuum in an incandescent lamp ? 

2. Compare the carbon, metalHzed carbon, tantalum, and tungsten 
lamps. 

3. What is the principle of operation of the Cooper-Hewitt tube ? 

4. How does the Moore tube differ from the Cooper-Hewitt tube ? 

5. Which has the better light distribution, a tube illuminant or 
an incandescent lamp ? 

6. Explain the process of manufacture of an incandescent lamp 
filament. 

7. What is the object of using a Holophane reflector on an incan- 
descent lamp ? 

8. Why is a tantalum lamp preferable to a carbon lamp where the 
voltage is likely to be below normal ? 

9. Compare the color values of the various incandescent illuminants. 
10. Why is it necessary with a Cooper-Hewitt lamp to tilt the tube 

at starting ? 



CHAPTER XIII 



RECORDING WATTMETERS AND THEIR USE 



Engineering practice has recognized that for commercial 
work the motor type of meter is the most suitable. The 
Edison bottle meter was used for many years. This 
device consisted of two zinc plates 
placed in a solution of zinc sulphate 
shunting a standard resistance, suitable 
resistances being placed in the circuit of 
the cell. The operation of this meter 
depended upon Faraday's law that the 
\veight of metal deposited in a given 
time is directly proportional to the cur- 
rent used. Since the constants of the 
cell were known, it was therefore pos- 
sible to calculate the energy used by a 
consumer by simply weighing the elec- 
trodes at regular intervals. Theoreti- 
cally the meter was very accurate, but 
was finally discarded owing to the fact 
that the consumer could not read the 
meter, and that mistakes arose from im- 
proper weighing of electrodes and from 
rough handling of the cells. The motor 
type of meter (see Fig. 291 for Thomson 
Recording Wattmeter) consists of a 
motor generator. The motor element fig 
of the Thomson meter, old style, con- 
sists of a high resistance armature. Fig. 283, with no iron in 

221 




283. — Armature of 
T.R.W. 



222 



EXPERIMENTAL ELECTRICITY 



its circuit, connected through an additional resistance, Fig. 
284, and compensating coil, Fig. 285, to the pressure ter- 
minals. The field coils. Fig. 286, 
having no iron in their circuit, are 
placed in series with the line. 
Owing to the absence of iron in 
either armature or field circuit, 
the field strength will vary directly 
as the current passing through the 
circuit, and the armature strength 
will vary as the potential in the 
circuit varies. The torque, or 
twisting effect of the armature, 
will therefore be proportional to 
the product of the armature and 
the field current, or to the arma- 
ture pressure and the field current, the armature current 
being proportional to the pressure. As there is no iron in 




Fig, 284. — Armature Resistance 
T. R.V^. 




Fig. 285. — Compensating Coil. 



the magnetic circuit, there is present practically no varia- 
tion in flux due to saturation and no counter e. m. f. Under 
these conditions there would be a tendency for the speed of 




RECORDING WATTMETERS AND THEIR USE 223 

the meter to keep increasing for a given energy input. 

This is Umited, however, by the generator action produced 

by rotating a disc between the 

poles of magnets, Fig. 287, 

placed in the bottom of the 

meter. As previously stated, 

when this disc rotates between 

the poles of the magnet, an e. Fig. .86. -Field Coil. T.R.w. 

m. f. is generated in the disc. This e. m. f. is directly pro- 
portional to the speed of rotation of the meter, as the field 
strength of the magnets is constant. 
The e. m. f. causes a current to circu- 
late in the disc, and this current pro- 
duces a magnetic field. The current 
which circulates is proportional to the 
FIG. 287. -Magnets, Type c g. m. f. generated in the disc, since 

Meter. , ° ^ , ,. . . 

the resistance of the disc is practi- 
cally constant. If the temperature of the armature, the 
field coils, or the disc should change during operation 
at heavy load, these values would change slightly. In 
practice this does actually occur to a slight degree. As 
the disc is equivalent to a conductor short-circuited upon 






r. R. W. Gear. Fig. 289. — T. R. W. Gear. 



itself, it is obvious that the load upon the meter will vary 
directly with the speed. By the combination of this 



224 



EXPERIMENTAL ELECTRICITY 




RECORDING WATTMETERS AND THEIR USE 225 



variable speed load with the speed characteristic, and the 
torque of the armature being proportional to the energy 
input, the speed of the meter 
becomes directly propor- 
tional to the energy input. 
The speed of the meter shaft 
is transmitted to a chain of 
gears, Figs. 288, 289, record- 
ing the revolutions upon a 
dial. With the later kind of 
Thomson Recording Watt- 
meter, known as the Type 
C meter, Figs. 290, 291, many 
improvements have been 
made over the old type of 
Thomson Recording Watt- 
meter. The chief of these 
has been the reduction in 
friction. This has been ac- 
complished by reducing the 
weight of the moving element and the diameter of the 
commutator of the meter armature, by substituting gravity 
^^ brushesfor the old type of spring 

H^2 brush, Fig. 292, by providing a 
^^^T ball-shaped armature and cir- 

^^^r cular field coils, and by reduc- 

^Hg^^^^ ing the air gap and the amount 

^^^^" . ^^ of copper required. The meter 

iS'""' " ■ is supported by three screws in- 

stead of the four used in the 
old type of meter, one of the 
supports being central so that through gravity the meter 
will hang plumb. Since the covers of these meters are 




Fig. 290.— Type C, T. R. W. 



Fig. 292. — Spring Brushes, 
T. R. W. 




226 EXPERIMENTAL ELECTRICITY 

removable from the front instead of from above, as with 
the old T. R. W., Fig. 293, the meters may be placed in 
small places. The wires enter from 
each side instead of from the bottom 
and are protected at their inlet points 
with pieces of felt to prevent dust from 
entering the meter. The compensating 
coil, the front magnet, and the field coil 
may be removed, readily permitting ac- 
FiG. 29^— CoveVof Old cess to the armature. The frame of the 
Style T.R. w. meter is made of aluminum and is split, 

insulating material being inserted in the gap so as to pre- 
vent any possible transformer action when the meter is used 
on alternating currents. The armature is wound with a 
special wire of dipped insulation. When this wire was 
first used, it was found to rub off easily, but the process 
has been improved. This type of insulation permits of 
using a very compact winding. The adjustment of the 
compensating coil is radial, with the Type C meter, and 
therefore allows of closer adjustment than with the old 
type of meter. The magnets of the meter are arranged 
so that the armature is readily accessible. 

Friction of Recording Wattmeter. — The friction of a re- 
cording wattmeter plays an important part in its accuracy. 
Every precaution is taken in the initial design of a meter 
to reduce the friction to a minimum. The movable element 
is set upon a steel pivot moving in a jewel bearing. Sap- 
phire jewels are used for the low capacity meters and dia- 
mond jewels for the larger capacity meters. With the initial 
friction thus reduced to a minimum, means must be taken 
to overcome or compensate for the remaining friction. This 
is accomplished by means of the compensating coil, Figs. 
285-290, which is placed in series with the potential circuit 



^AA- 



RECORDING WATTMETERS AND THEIR USE 22/ 

of the meter. The action of this compensating coil is some- 
what analogous to a series motor, having its own field act- 
ing on the armature field. This coil is shifted till its field 
just compensates for the friction of the meter. In other 
words, it is adjusted until the armature just does not turn 
round. Any variation in the friction of the meter, such as 
vibration, will obviously cause the meter to rotate slightly, 
although no current be passing through the fields. 

Experiment 137. Connect a recording wattmeter up to the circuit 
without placing any lead upon it. Fig. 294. Vary the compensating 
coil, noticing that the armature may be 
made to rotate when the position of the ,,5 ^^^^^ 
compensating coil is near that of the ''•'^• 

armature. Set the compensating coil 

, , . .,, Fig. 294. — Wattmeter Circuits, 
so that the meter armature will not ro- 
tate. Strike the base of the meter .^ ,^h/^ 

rapidly several times and notice the us volts \ 

movement or (;>r^^//;/_g' of the meter. It is ^' ^ 

very important in locating meters to see ^,^ ,„ .. . r^- 

■> r & ^ Fig. 295. — ^\attmeter Circuus. 

that they are not placed where there is 
much vibration. 

Experiment 138. Connect up a recording wattmeter, as in Fig. 295, 
to load. Vary the load upon the meter in i6-candle-power equivalents, 
and notice that the speed of the meter is directly proportional to 
its energy consumption. 

In Fig. 296 are shown a set of curves obtained by Mr, 
Eichert, of the Edison Illuminating Company of Brooklyn, 
which show how the compensating coil reduces the 
friction loss in the meter. These curves also show how, 
with the induction meter, whose moving element consists of 
nothing more than a disc in which exists a rotating field, 
the friction loss is almost zero %, due to the small weight 
of the moving element. 

Testing Meters. — Three methods are in common use for 
the testing of meters : 



228 



EXPERIMENTAL ELECTRICITY 



I. Direct method, the indicating wattmeter method, or 
the volt ammeter method. 

2. The rotating stand- 



CALIBRATION CURVE " 
G-E.COS. TYPE 'c'V 

WATTMETER y_ 

WITH COMPENSAT-w 

ING COIL = 



FRICTION CURVE " 

^ 90 
G.E.CO'S. TYPE "C"ii. 
O 
WATTMETER ,_ 80 

WITHOUT COMPEN-^ 70 
SATING COIL |?i 



CALIBRATION CURVE • 

G.E.CO'S. TYPE "l "1 

WATTMETER ! 



FRICTION CURVE 
G.E.CO'S. TYPE "i ' 

WATTMETER 



PERCENT OF LOAD 

70 80 90 



T 


L_ 


Y- - - 


- 1 




° 




1 


1 


(,: \ _± 


PERCENT OF LOAD 

10 20 30 40 50 60 70 80 90 10r 


"" :::-:::::::_:xi 


^ J, ^ 




' "/ 


3 ^ 






Pl 


L_: L : 


PERCENT OF LOAD 

10 90 ao 40 50 60 70 80 90 10 


.11 1 ■ " 


0^ 




" 1 








° 




PERCENT OF LOAD 

10 '20 30 40 60 bO 70 80 90 10 


or"i:::"i — x~ 


j^^ -^ 


D (« H 




° 






r" 





Fig. 296. — Friction Curves of Meters. 



ard method. 

3. The standardized 
resistance method. 

The Direct Method.— 
This is the most accurate 
of the three methods, pro- 
vided that a steady 
load is obtainable. This 
method requires the use 
of three instruments and 
a larger amount of time 
to test meters than some 
of the other methods, 
and it results in slight 
errors in observation on 
variable load. On the 
other hand, this method 
has certain advantages. 
It is comparatively easy 
by means of it to deter- 



mine the exact test load ; the voltmeter may be also used 
to locate grounds and armature trouble; the standard volt- 
meter, ammeter, stop watch, etc., need to be checked less 
frequently than other standards, and they indicate any 
change in the meter under test. The instruments are not 
so likely to be affected by stray fields from the leads of 
the meter under test. 

Experiment 139. Connect up a recording wattmeter to a source of 
potential, Fig. 297, placing a load upon the meter, and arranging an 
ammeter in series and a voltmeter in parallel with the meter. Close 




RECORDING WATTMETERS AND THEIR USE 229 

switch A^ noting the time with a stop watch, and also count the revolu- 
tions of the disc. After the meter has been operating for a time open 
switch A- and caHbrate the meter. 
The constant marked upon the disc 
for small-sized meters is the watt hours 
per revolution of the disc. Multiply 
the revolutions of the disc by the disc ^^^- "97- - Calibrating a Meter. 
constant in order to obtain the watt hours. The volts indicated multi- 
plied by the amperes by the time indicated give the watt seconds, and 
this amount divided by 3600 will reduce the quantity to watt hours. 
Compare the two sets of values of watt hours. 

Rotating Standard Method. — The rotating standard was 
first developed by Mr. Mowbray of the Brooklyn Edison 
Company. This meter is in reality a standard recording 
wattmeter which is connected in circuit with the meter 
under test. Upon the top of the meter is a graduated 
dial, over which moves a pointer attached to the spindle 
of the meter, indicating a fraction of a revolution of the 
disc. The test meter and the meter under investigation 
are both compared with a given number of revolutions. 
Different sets of field coils may be used when testing at 
normal load, at full load, and at Hght load. The per- 
centage accuracy of the meter being tested is obtained 
from the following simple relation : 

actual revolutions X 100 ^ r . ^ 

: = percentage of accuracy of meter. 

allotted revolutions 

The advantages of this type of meter are its simplicity 

and rapidity of testing. It requires but one man to make 

tests and practically eliminates observation errors, recording 

accurately on variable load. The disadvantages of this 

type of meter are the necessity of checking the standard 

meter daily, the inability to determine shght changes in 

accuracy of the meter under test, the liability of the meter 

to be affected by stray fields from its own leads, and the 



230 EXPERIMENTAL ELECTRICITY 

chance of error due to not properly heating the potential 
circuit. 

The field coils of the meter may be made up in four parts 
as in the G. E. rotating standard. These coils may be 
connected in series or in parallel so as to obtain approxi- 
mately the same ampere turns, resulting in the same torque 
for various loads. For instance, if 40 amperes are passed 
through the field coils connected all in parallel, 10 amperes 
will pass through each coil. If 20 amperes are being used, 
the field coils may be connected in series parallel arrange- 
ment of two sets of two coils in series, each set of coils 
consisting of two coils in parallel. This would likewise 
send 10 amperes through each field coil, as in the previous 
case. If 10 amperes are being tested, all of the four coils 
may be placed in series, in which case 10 amperes pass 
through each field coil. As the potential circuit is wound 
of copper wire, it must first be connected to the circuit for, 
say, ten minutes at normal load to warm up. Sometimes 
100% overload is placed upon the potential circuit for a 
short period, as the heat generated varies as the square of 
the current, = I'^Rt x .24 expressed in gram-calories. A 
recent improvement made by Mr. Mowbray consists in 
increasing the ampere turns of the field and decreasing 
the ampere turns in the armature. An external resistance 
having a negligible temperature coefficient is placed in 
series with the armature to bring it to normal value. This 
reduces to a marked extent the error introduced by a 
change in the resistance of the armature circuit. 

Experiment 140. Place a rotating standard in series with a service * 
meter, and check the service meter. 

Standardized Resistance Method. — This method consists 
in the use of a standardized resistance, Fig. 298. A volt- 
meter is used in connection with the resistance to indicate 



RECORDING WATTMETERS AND THEIR USE 23 1 




Fig. 298. — Standardized Resistance used by the Boston Edison Company. 



232 



EXPERIMENTAL ELECTRICITY 



the potential. By means of a calibration curve it is possi- 
ble to determine the load when the voltmeter reading is 
known. The load may be varied by a simple arrangement 
of switches. The resistances may be wound so that they 
will be suitable for use on both direct and alternating cur- 



i 


Wk 


%aM 


^l^^^^i^S^^m^Sk 




^H 


HI 


.^^^p 


ipppfT' 


'^^^^Mt 



Fig. 299. — Load Box, Boston Edison Co. 



rent. The resistance wire has practically a zero tempera- 
ture coefficient, eliminating temperature errors. The 
resistance is connected across the meter similar to a regu- 
lar live load, being in series with the field coils of the 
meter. Care must be taken with this method to have the 
leads sufficiently large between the meter and the resist- 
ance so as to minimize errors due to drop in potential in 
the leads. Voltmeters should be connected as near the 
potential point of the meter and the standard resistance as 



RECORDING WATTMETERS AND THEIR USE 233 

possible. This method is used in conjunction with other 
tests by the Boston Edison Company, the New York 
Edison Company, and the Philadelphia Edison Company. 
One form of load box is shown in Fig. 298. It weighs 4.5 
pounds, and has a capacity of 10 amperes. A very com- 
pact and satisfactory load box used in testing meters is 
that shown in Fig. 299, recently developed by the Boston 
Edison Co. This resistance opens out fanlike ; it is very 
compact, and weighs but 3.5 pounds, and as the weight of 
the case is 2.75 pounds, the total weight of the load box is 
6.25 pounds. It has a capacity of 60 amperes at 113 volts, 
and is made up in sections. Owing to its appearance, it is 
termed by the meter testers a bed resistance. 

Installation Tests. — When a meter has been installed 
on the customer's premises, it is necessary before putting 
the meter into service or placing the fuses in position to 
make a few simple tests, as the customer's wiring may be 
short-circuited or grounded, or the field coils of the meter 
may be improperly connected. These tests are best illus- 
trated by the following simple experiments. 

Experiment 141. Let A, Fig. 300, be the fuse plug on a cus- 
tomer's premises on the service side of the meter, and let B be the fuse 
plug on the house side. Place the fuses + — 
in the service side of the meter A^ ne volts 

and place an incandescent lamp in the '"'^^ 

positive fuse plug on the house side B. 

If the lamp Hghts, the service is Fig. 300. -Service Tests of Meter. 

grounded. If the lamp does not hght place another lamp in the other 

fuse plug at B, taking care that the customer's lamps are turned off. 

If the lamps light at half candle power the customer''s wiring is 

short-circuited. 

With a three-wire meter one field coil is in series with each 
of the outside legs of the circuit. When the system is bal- 
anced, the current passing through each field coil will be the 



A 



05 



— i-WVWVS 



234 EXPERIMENTAL ELECTRICITY 

same in magnitude. If the system is unbalanced, a greater 
current will flow in one leg of the circuit than in the other leg, 
the neutral wire carrying the difference in current on both 
sides of the system. When the field coils are properly 
connected up, the magnetic circuit of each pair of field coils 
will help the other along. If the field coils should by any 
means be reversed, the fields will work in opposition to 
each other, the side having the stronger field dominating 
the direction of motion of the meter. 

Experiment 142. Connect up a three-wire meter to a three-wire 
load such as a lamp board, and reverse one of the fields, Fig. 301 . Vary 

^_ the load upon the meter, first having one 

- side and then the other carry the greater 
-, load. Notice that the meter will rotate 

I 1 first in one direction and then in the 

Fig. 301. — Field Coils of Meter other. This experiment is very impor- 
improperly connected. ^^^^^ ^^ illustrating the fact that it is im- 

possible by merely noting the direction of rotation of a meter when 
installed to determine whether or not the field coils are properly con- 
nected. If the direction of rotation is to be taken as an indication of 
correct installation, some idea of the distribution of the load must 
also be had. 

Inspection Tests. — When a meter has been in use for 
some time, it is necessary to test it. The frequency of 
these tests depends upon whether a direct current meter 
or an alternating current meter is in question and also upon 
the capacity of the meter. The schedule used by one of 
our largest lighting companies is as follows : direct current 
meters up to and including 25 amperes capacity require 
one test a year ; those of from 50 to 100 amperes require two 
tests a year; those of 100 amperes and above require four 
tests a year. Alternating current meters, single phase, are 
tested as follows : those of all capacities require one test 
every two years ; polyphase meters of all capacities require 



RECORDING WATTMETERS AND THEIR USE 235 

two tests a year. The meters of special customers who 
use the current 24 hours a day are tested once a month. 

How to make Tests. — Meters should be tested on three 
loads, — on light load, on normal load, and on full load. For 
light load 10 % of the meter capacity is used, for full load 
from 50 to 100 % of the meter capacity is used, for normal 
load from 20 to 100 % of the customer's installation or 
commercial load is used. The Public Service Commission 
of New York recommends the following schedule in de- 
termining the normal load from the connected load : 



NORMAL LOAD 



Class 


Type 


% OF Installation 


A 


Residences and Apartments 


25 


B 


Churches and Offices 


45 


C 


Elevators 


20 


D 


General Stores 


60 


E 


Signs 


100 


F 


Blowers 


100 


G 


Theaters 


60 


H 


A and C 


25 


I 


^and C 


40 


K 


CandZ> 


50 


L 


D and E ox F 


70 


M 


f Motors except C and F 1 
[Arc Lights J 


Load in use, 

otherwise 

50 



Having tested a meter on each of these loads, the 
average per cent accuracy may be found by taking the 
light load A, plus the full load B, plus 3 times the normal 
load C, and dividing by 5, giving the average per cent, 

— = average per cent. 



5 



236 



EXPERIMENTAL ELECTRICITY 



Where alternating current meters are to be installed on 
inductive load, they are tested on a 60 % power factor in 
the testing room before being installed. 

Load. — While it is possible to use the customer's load 
as a test load in checking a meter, this is undesirable and 




Fig. 302. — Typical Meter Installation, Alternating Current Service. 



inconvenient. Various loads, such as portable resistances, 
water rheostats, lamp boards, load boxes, etc., may be used. 
The customer's load is shunted while the meter is being 
tested. A small set of storage batteries may be used for 
testing meters, such as 500-volt meters, using a carbon 
rheostat to regulate the current. Small and compact step- 
down transformers may also be used for testing alternating 
current meters, employing the transformer as a load. 



RECORDING WATTMETERS AND THEIR USE 237 



^mm 


^^^IHii^B 


p 


i 


i 


L 


a 










\ 


Hi 


.j\ 


^ 


n 


i^ 


Wi 




A 


j^ 


JliM^lgii^ 






i-! 


•s. 


1 • 


J 


Fl 


P 


""g^ 




'^ySS 


p *•'* 


^ 


1 


s 


iw M?" 


i 


fe 


11 






'■ 




jj 


■ 


I^'h 


1 


hH 


1 


1 




1 






1 




f . m 








"™I^ 








' 1 


^s 


r 








.- 






















L'.- 










i 


^M^ 


^ai 


toi[^;.r-^ 




V 



Fig, 303. — Typical Service of Meters. 




Fig. 304. — Combination Direct Current and Alternating Current Service. 



238 



EXPERIMENTAL ELECTRICITY 



Meter Installations. — The capacity of a meter to be in- 
stalled in any place depends upon the size and character 
of the connected load. Meters with few exceptions have 




Fig. 305. — T. R. W. and Type C. Alcter Sli\ 



a smaller capacity than the connected load, as overmeter- 
ing not only increases the cost of meter installations, but 
results also in a loss to the company. This is due to the 



RECORDING WATTMETERS AND THEIR USE 



239 



fact that the meter with 
a small capacity will 
register a small load 
more advantageously 
than a larger capacity 
meter, the tendency for 
the larger meter being 
to run slow on light 
loads. As a rule, a 
meter of 50 % of the 
customer's capacity is 
satisfactory, for it is 
seldom that more than 
one half of the installa- 
tion is in service -at one 
time, except perhaps 
when some special func- 
tion is occurring. One 
manager has said that 
it is good economy to 





Fig. 306. — Typical Direct Current Service. 



Fig. 307. — Single A. C 
Service. 



burn out a meter occasionally. Where, 
as in signs, all of the customer's lamps 
may be lighted simultaneously, the meter 
should be of the same capacity as the 
installation. This is also true of some 
motor loads. The table on page 235 
gives some idea of the relation between 
the connected load and the normal load. 
Care .must be taken to consider the 
overload capacity of a meter in all cases. 
The starting current may be excessive 



240 



EXPERIMENTAL ELECTRICITY 



compared with the installed capacity of the load, as with 
motors on elevator service. In such cases the motors may 



mmmm^^^ 



:^-.„I.__ 






mm^i^m^^^<^^^^^ 







Fig. 308. — Typical Meter Installation. 

be started and stopped frequently, and a capacity 25 % 
greater for the meter than the installed capacity of the 
motors may be necessary. 



RECORDING WATTMETERS AND THEIR USE 24 1 

Meter Wiring. — In addition to the ordinary require- 
ments in the installation of a meter, such as accessibility 
and absence of dampness and vibration, the meter should 
be installed so that its position will be permanent, and so 
that it will be safe and cannot be tampered with. Formerly 
too little attention was devoted to this phase of the subject. 
Recently, however, considerable advance has been made 
in this direction by housing the meter in sheet-iron boxes 
and the service switches in metal boxes, and inclosing the 
immediate wiring in the vicinity of the meter in a conduit. 
Specimens of this new development in meter installation 
are shown in Figs. 302-308, as developed by Mr. J. W. 
Lafferty for the Brooklyn Edison Company. 

QUESTIONS 

1. Give the principle of operation of the recording wattmeter. 

2. What is the function of a compensating coil ? 

3. Why does a short circuit on the system often alter tlie accuracy 
of the meter ? 

4. In making meter tests, why is it important not to liave the test 
leads too near the meter ? 

5. Give the principal methods of testing meters, with their relative 
advantages and disadvantages. 

6. What is meant by the light load adjustment of a meter, and 
what is meant by the full load adjustment ? 

7. How would you test the jewel of a meter to see whether it is 
perfect ? 

8. What is meant by the creeping of a meter and how may it be 
obviated ? 

9. W^hat advantages does the motor type of meter possess over an 
electrolytic meter for commercial work? 

10. How may the jewel of a meter be protected during transportation ,'' 



CHAPTER XIV 
ELEMENTARY PRINCIPLES OF ALTERNATING CURRENTS 

An alterjiating ciLrrent differs from a direct current in 
that an alternating current continually reverses its direc- 
tion of flow with a regular periodicity. An alternating 
current starts from zero, flows in one direction, the current 
gradually rising from zero to a maximum value, and then 
falling again to zero. When it reaches zero, it changes 
its direction of flow, rising to a maximum again, and falling 
again to zero. This fluctuation of the current, flowing first 
in one direction and then in another, continues with a 
definite sequence, the sequence being very rapid. For 
normal practice it may be 25 or 60 times a second. The 
period, or complete cycle of operations, is so rapid that arc 
lamps and incandescent lamps may be operated upon it 
without an inexperienced observer being able to tell 
whether the current supply is direct or alternating. In an 
alternating current circuit two other factors besides resist- 
ance^ namely, inductance and capacity, affect the operation of 
the circuit. Both of these factors are also present in a direct 
current circuit, but they come into play in the direct current 
circuit only when there is any change in the current flow. 
Thus, when a direct current circuit is closed, the current 
does not rise instantly to its normal value if inductance is 
present in the circuit, but a definite time interval elapses 
before the current reaches normal value. This time inter- 
val is quite short for most circuits, but in a large generator 
it can easily amount to one second. This point can be 

242 



PRINCIPLES OF ALTERNATING CURRENTS 



243 



readily demonstrated by closing and opening a field switch, 
leaving it closed for various small time intervals. At first 
a small spark will occur which will increase with increased 
time intervals to an arc, A better conception of an alter- 
nating current circuit may be had from the following 
simple experiment. 



.MAXIMUM AT 14 AMPERES 




Fig. 309. — Instantaneous Current Curve. 

Experiment 143. Connect a dry battery of 1.4 volts potential to a 
resistance of I ohm. According to Ohm's law, 1.4 amperes will pass 
through the circuit. Gradually increase the number of cells, one at a 
time, until 10 are in circuit, and the current will increase to 14 amperes. 
(This will be only approximately true, as the internal resistance of the 
cells has not been considered ; but if a storage battery is used, it will be 
much more exact.) Then gradually decrease the number of cells in 



nections so that when the circuit is again closed the current will flow in 
the opposite direction, and continue the process as before, the currents 
rising to a maximum and then falling. If we plot a curve for this 
operation in terms of the number of changes and the current value ob- 
tained for each change, we shall obtain a curve like Fig. 309. This 
curve has five distinctive points, A, B, C, D, E. Points A, C, and E are 
zero values, and points B and D are maximum values. Points above the 
line may be called positive, and points below the line may be called 
negative. This complete operation of positive and negative values is 
termed a cycle, and the curve may be termed a curve of alterjiatmg 
cicrreiit. In an alternating current generator, the changes in current 



244 



EXPERIMENTAL ELECTRICITY 




flow are not so abrupt or irregular, as in Fig. 309, the change being uni- 
form and smooth, following a sine law, as in Fig. 310. When a coil 
of wire is wound upon an armature and rotated in a magnetic field 
(Fig. 311), the terminals of the coil being 
connected to slip rings, which in turn 
are connected to a resistance, such as a 
lamp, the current which will pass through 
the circuit will be somewhat of the shape 
of Fig. 310. This is due to the fact that 
the magnetic flux which passes through 
the armature is greatest at the centers of 
the poles, and is practically zero between the 
poles. The same effect may be illustrated 
FIG. 310. - Sine Curve. -^^ ^^^^j^^^ manner. 

Experiment 144. Take a solenoid 

of a large number of turns (Long Tom), 

and connect it to the terminals of a 

galvanometer, as in Fig. 312. Arrange 

a bar magnet so that it can be moved 

in and out of the coil, causing the 

pointer of the galvanometer to deflect, 

first in one direction and then in the 

opposite direction, due to the e. m.f. 

generated in the coil. It will also be ' ^^^' 

noticed that when the magnet is plunged into the coil, 
the needle will deflect in one direction, and when it is 
withdrawn from the coil, it will deflect in the opposite 
direction. If the speed of motion of the magnet is 
increased to a maximum, and then decreased to zero in 
the forward motion, and if this occurs likewise in the 
return motion, a series of deflections will result. If it 
were possible to plot these, they would fall on a curve 
similar to Fig. 310. 

The Contact Maker Electro-motive force 

and current curves for actual machines in 
practice may be obtained by means of contact 
Fig. 312.— Gen- ^g^^gj.^ operatins: in coni unction with some 

erating an Alter- ±0 j 

nating E. M. F. mcans of balancing and of reading the poten- 




Generation of E. M. F. 






i 





PRINCIPLES OF ALTERNATING CURRENTS 245 

tial. The contact maker, Figs. 313, 314, consists of two 

metal rings mounted one over the other, with insulation 

between, on the end of a shaft, 

which is supported in bearings p 

and connected through a flexible hj 

couplingtotheshaft of the machine |_ 

being tested. In the outer ring is Fig. 313. —Contact Maker. 

bored a hole, through which passes 

a circular rod connected into the 

inner ring, passing through the 

insulation between. The hole 

through the outer ring must be of ^^^- 3i4. - Contact Maker. 

sufficient size so that an air space will surround the rod, 
insulating it from the ring. The top of the rod is flush 
with the surface of the outer ring. A copper brush presses 
against the top surface of the outer ring, making electrical 
contact between the two rings as the contact maker rotates. 
The brush is mounted upon a movable ring which is 
graduated, the ring turning about the same axis as the 
contact maker. It is thus possible to set the contact 
maker so that it will close the circuit at any point of a 
revolution, the exact position being readable on the vernier 
scale attached. Pressing against the side of the outer 
ring is a brush also, which completes the circuit of the con- 
tact maker, as the inner ring is of the same potential as the 
frame of the machine. The brush pressing against the 
outer ring may be mounted upon an insulated arm, so that 
the brush will simply be the push button of the circuit. 
In taking a series of observations, the contact maker 
brush is set at various positions, such as every 5°, or 
(360/5 =) 72 readings for one revolution. These degrees, 
however, are mechanical degrees and not electrical degrees, 
except where a two-pole machine is used. If a four- 



246 



EXPERIMENTAL ELECTRICITY 






il 



pole machine were used, each mechanical degree would 
equal two electrical degrees ; if an eight-pole machine, 
each mechanical degree would equal four electrical degrees, 

etc. Each pair of poles is 
equivalent to 360 electrical 
degrees, as shown in Fig. 309. 
Bearing in mind that the po- 
tential of the generator at the 
particular fraction of a second 
that the circuit is closed by 
the contact maker may be 
either positive or negative, 
and of any magnitude from 






GRAPHITE 
ROD 



TELEPHONE 
RECEIVER 



REVERSING 
SWITCH 



X 



tl 



S^—^ 



CONTACT 16 C. P. 

MAKER LAMP'. 



FIG. 315. -Using a Balance Set to zero to a maximum, the ques- 
obtain E. M. F. Curve. ^^^^ ariscs as to how this poten- 

tial maybe measured. In Fig. 31 5 is shown a ii6-volt di- 
rect current circuit connected through two i6-candle-power 
lamps to a carbon rod which has a high resistance, about 
3000 ohms. When the circuit is closed, the lamps will 
not hght up, but will have a resistance of from 400 to 500 
ohms each. Assuming a com- 
bined resistance for the lamps 
of 1000 ohms, and a resistance 
of the rod of 3000 ohms, it 
is evident on a ii6-volt direct 
current circuit that the poten- 
tial would distribute itself over 
the circuit in the proportion of 
29 volts for the lamps and Zj 

volts for the rod. If 240 volts fig. 316. -Method of Balancing 

were connected across the circuit, ^- ^' ^•*^- 

the potential would rise across the rod to over 200 volts, as 

the lamps would partially light up, their resistance falling 




PRINCIPLES OF ALTERIVATIIVG CURRENTS 247 

quite perceptibly. If one terminal of the rod is connected to 
a voltmeter, then to a telephone receiver, and then from the 
telephone receiver to one pole of a switch, and another slid- 
ing contact on the rod is connected to the other terminal 
of the voltmeter, and then to a i6-candle-power, through 
the lamp to the other terminal of the switch, we shall 
obtain an adjustable potential circuit, which may be 
connected through the contact maker to two slip rings of 
the alternating current generator. The switch in the 
circuit should be of the reversing type so that the direction 
of the direct current can be changed when desired. Read- 
ings taken with the switch in one position may be termed 
positive values, and readings taken with the switch in the 
opposite direction may be termed negative values. Sliding 
the contact along the graphite rod will give a variable 
potential, whose value will be indicated on the voltmeter. 
The alternating e. m. f . for a given setting of the con- 
tact maker will have a finite value, as previously stated, 
of from zero to its maximum value, and will be either 
positive or negative. Assume that the value to be measured 
is 40 volts positive. Fig. 316. To balance this potential 
in the circuit so that no current will flow either into the 
generator or out of the generator at the instant that the 
contact maker closes the circuit, a potential of 40 volts 
positive direct current must oppose it. When a balance 
is obtained, no current will flow through the telephone 
receiver, and no sound will be heard ; if a balance is not 
obtained, the click of the contact maker will be heard. 
To balance, the main switch is closed, and the telephone 
receiver is placed to the ear, the sliding contact being moved 
over the graphite bar. If the switch is in the wrong 
position, the sound in the receiver will be intense, due 
to the fact that the direct current will be trying to 



248 EXPERIMENTAL ELECTRICITY 

assist the alternating current. As the contact is moved 
along, the sound in the receiver, that is, the character- 
istic click, will gradually diminish in intensity, until 40 
volts is reached in this case, when the. sound will be 
either zero or at a minimum. On sliding the contact 
beyond this point, the sound in the receiver will increase. 
A little practice in the handling of this apparatus will 
enable an observer to estimate his value before he makes 
a balance, and to know whether his curve is rising or fall- 
ing, and when he is crossing zero ; in the latter case, he 
will need to throw his reversing switch. 

Experiment 145. Make a set-up as described above, and obtain a 
potential curve for an alternating current machine. If the alternating 
current machine happens to be a rotary converter, it will be necessary 
to connect a i : i transformer in the generator leads coming to the con- 
tact maker, before this same direct current potential, upon which the 
machine is operating, can be used to balance. When the readings are 
obtained, plot a curve using degrees for the horizontal axis, or abscissa^ 
and voltage readings as indicated on the voltmeter as the vertical axis, 
or ordinate. 

Experiment 146. Place a non-inductive resistance in the generator 

circuit such as a standard ohm and a lamp board, permitting about five 

amperes to flow through the generator circuit. 

-1]_lLj- — . Use an additional double throw switch, connected 

into the circuit, as in Fig. 317, so that simultaneous 

^aJ pressure and current curves may be obtained. 

Plot both sets of values on the same sheet, and 



a°pTratus notice that maximum and zero values of both 



t /(^ current and pressure values correspond, these 

Fig. 317. — Balance values occurring at the same time intervals. 

Apparatus. Qscillograph. — The oscillograph, one 

type of which is made by the General Electric Company, may 
be used to obtain the wave shapes of current and potential 
circuits ; it has an advantage over the contact maker in that 
the wave shapes may be made visible if the phenomena are 



PRINCIPLES OF ALTERNATING CURRENTS 249 

recurring, and in that they may be photographed if the 
phenomena are either recurring or transient. The princi- 
ple of the apparatus is quite simple. It consists of a flexi- 
ble suspension in the form of a loop of fine wire mounted 
between the poles of an electro-magnet. Upon this suspen- 
sion is mounted a small mirror about ^g- inch square. This 
suspension is connected through resistance to the circuit 
about to be measured, the amount of resistance in the cir- 
cuit of the electro-magnets and in the suspension circuit 
being determined by the amplitude of the deflection de- 
sired. The current through the electro-magnets should be 
about 1.5 amperes as a maximum. A ray of light is sent 
into the oscillograph box through the side, where it strikes 
a 45° prism, the beam being turned at right angles where 
it is focused upon the mirror. The arc lamp should 
be operated with small millimeter cored carbons so that 
the arc will remain steady. Be sure at any rate that the 
upper carbon, or positive, is cored. The light reflected 
from the mirror comes back through a slit in the back of 
the oscillograph, where it moves over the film as it rotates. 
For visual use a small induction motor on the side of the 
oscillograph rotates a sector and also vibrates a prism so 
that the rays are reflected vertically where they may be 
observed on the observation plate. Since the motion of 
the prism is about an axis at right angles to the motion of 
the mirror, the curve is spread out so as to be visible. 
The oscillograph is arranged usually with three separate 
suspensions so that various current pressure and time re- 
lations may be observed. When the film carrier is in use, 
an electrical contact on the axle of the carrier opens and 
closes the shutter, regulating the exposure as desired. 
Ordinary photographic films may be used, a 6-exposure 
film making two exposures for the oscillograph. Care 



250 EXPERIMENTAL ELECTRICITY 

should be taken in loading the film carrier to be sure that 
you have the emulsion side of the film outward and to 
overlap the film so that, when it is rotated, the top flap of 
the film will move past the exposure opening in the oscil- 
lograph and not catch in the opening, as it is likely to do 
if the film has been mounted wrong. In using the oscil- 
lograph, since there are usually six switches which have 
to be closed before an exposure is made, it is well to write 
out the series of steps closing the switches in i, 2, 3, 4, 5, 
6, order. This is especially important where one person 
only is operating the oscillograph. 

Effective Values. — In an alternating curve of instan- 
taneous e. m. f.'s. Fig. 310, there are various values which 
may be considered, — the zero values^ the maxifmLin val?ies, 
the average value, and the effective value. In commercial 
work we are interested in two values particularly, the maxi- 
mum value and the effective value. Most portable instru- 
ments, such as ammeters and voltmeters (Fig. 318), are 
constructed to indicate effective values. The effective 
value is known as the square root of the 7nean square 
value. The effective value of an alternating current cir- 
cuit is such an alternating current as would have the 
same heating value as a given direct current. Mathemati- 
cally, the maximum value divided by the square root of two 
gives the effective value. 

Effective value = maximum/V2. 
Effective value = maximum/1.41. 

To find the effective value for a given sine curve ob- 
tained from an alternating current machine by means of a 
contact maker, proceed as follows. Take a certain number 
of values — 20 of the original curve — and square them. 
Plot over the original curve a current squared curve, using 
the same abscissa but different ordinates. Divide up this 



PRINCIPLES OF ALTERNATING CURRENTS 



251 



curve into rectangles, and calculate as closely as possible its 
area. If a planimeter is at hand, it may be used to obtain 
the area. Having determined the area in square inches, 
divide the area by the abscissa, or the length along the zero 
line between two successive zeroes, and obtain the mean 
height of the curve in inches. Plot this value vertically 
on the curve, and determine its equivalent value in current 
squared units. This will then give the mean square current 




Fig. 318. Interior of Weston Alternating Current Voltmeter. 

value. Extracting the square root of this quantity will 
give the effective value, or the square root of the mean square 
value. An effective current value of 100 amperes alternat- 
ing current would have the same heating effect as 100 am- 
peres of direct current. The heating effect of a direct 
current circuit is thus expressed : 

U=PRt X .24, 

where Uis given in gram calories, /in amperes, R in ohms. 



2 52 EXPERIMENTAL ELECTRICITY 

t in seconds, and .24 h Joule s coefficient. One gram calorie 
is the amount of heat required to raise the temperature 
of one gram of water i°C. 

The relation between the maximum value and the 
average value may be given as follows : 

maximum = ir/2 x average = 1.57 x average. 

(For the mathematical proof of this equation, see Steinmetz, 
Alternating Current Phenomena, page 13. Trigonom- 
etry is used only in the proof.) 

Form Factor. — The form factor of an alternating cur- 
rent wave is the relation existing in the given wave 
shape between its average and its effective values. 
It is usually given in the form of the following ratio : 

F. F. = effective/average. 



In the sine curve the value becomes .707/.636 = i.ii. The 

value .707 =—-=. This 

V2 
curve may also be written 



value .707 = — =. This equation for form factor for a sine 

V2 



— -'-^max. 

V2 

F.F.= = I.II. 

^~E 

-'-^niax. 
TT 

Alternating Current Generators. — Alternating current 
generators are of three types : 

1. Alternators with a stationary field winding using a 
revolving armature. 

2. Alternators having a stationary armature and a 
revolving field winding. 

3. Alternators having a stationary armature and a sta- 
tionary field winding using a rotating element termed an 
inductor. 



PRINCIPLES OF ALTERNATING CURRENTS 253 




Fig. 319, — Single- 
phase Generator. 




Fig. 320. — Three- 
phase E. M. F. 



Type i is used for small machines, and consists in sub- 
stituting slip rings for the commutator of small direct cur- 
rent generators. A small two-pole motor 
may have two slip rings mounted upon its 
shaft on the opposite side of the armature 
to the commutator and tapped into the arma- 
ture winding at two points, as in Fig. 319. 
This produces a single-phase machine em- 
ploying two slip rings. To produce a three- 
phase machine, employing three-phase e. m. f.'s, as in Fig. 
320, three taps 120° apart, Fig. 321, con- 
nected to three slip rings, are used. 

Type 2. In this form of machine 
there are no slip rings except for the ex- 
citation of the revolving field winding, 
the armature which usually supplies high 
potential being well insulated and connected directly to the 
main line. 

Type 3. This type of machine. Fig. 322, termed an 
inductor generator, has likewise no slip 
rings for the armature winding, the latter 
being connected directly to the line. The 
principle of operation here employed is 
that a series of iron poles on 
the rotating element will 
first span one set of arma- 
ture and field windings lying 
between, and then lie in between the next 
set of windings, causing the flux to rise 
from a minimum to a maximum value with- 
out changing in sign. As the field winding lies in between 
the armature windings, the rise and fall of the flux gener- 
ates an alternating e. m. f. in the armature winding. In 





Fig. 321. — Three- 
phase Generator. 



Fig. 322. — Indue 
tor Generator. 



254 



EXPERIMENTAL ELECTRICITY 



Fig. 322 only one set of the armature windings is shown ; 
the field windings lying in back of these armature windings 
are likewise not shown. A complete description of these 
machines may be found in Sheldon's Alternating Currents, 
pages 136-139. 

Detailed tests illustrating the characteristic curves and 
performance of alternators may be found in Bedell's 
Direct Current and Alternating Current Testing, pages 
62-102. 

Capacity. — When a two-conductor cable is connected 
up to a direct current source of potential and no load 
is on the cable, a certain flow of electrical energy occurs, 
lasting for a short interval of time. If this cable be 
disconnected from its direct current 
source, and be allowed to discharge 
through a galvanometer, Fig. 323, a 
deflection of the galvanometer will oc- 
cur, lasting for some time. This de- 
flection is so severe on a service cable 
that it is customary to short-circuit the 
galvanometer at first and then to open 
the circuit after a certain time interval. This experiment 
indicates that a certain amount of energy 
has been stored somewhere in the cable. 
What actually occurs is that a displace- 
ment current flowing through the dialec- 
tric of the cable creates a bound charge 
in the dialectric and a free charge on the 
plates. The effect produced. Fig. 324, is 
termed electrical polarizatiojt, the latent 
charge in the dialectric being separated fig. 324. 
into positive and negative elements. But 
electrical polarization should not be confused with the term 



cable/qAv-^^ 



\(^ 



Fig. 323, — Experiment 
showing charging of 
Cable. 



- + 

- -h 

- + 

- + 

- + 

- 4- 

- -I- 

- -I- 

- + 

- -h 



Dialectric 
Polarization. 



PRINCIPLES OF ALTERNATING CURRENTS 255 

polarization as applied to an electrolytic cell. In the former 
case the two charges, positive and negative, are bound, being 
held in equilibrium, while in the case of the cell the positive 
and negatively charged ions are actually split apart, travel- 
ing to the electrodes of opposite polarity. The effect 
just described is termed the effect of capacity. It is to 
be found not only in cables, but in all electrical apparatus 
where two conductors are separated by a dialectric. Forms 
of apparatus consisting of conducting foil or plates sepa- 
rated by air, or by some dialectric, such as paper or glass, 
are used for various electrical purposes and are termed 
condensers. The unit of the capacity is the farad. A 
condenser whose potential can be raised to one volt 
by one coulomb (one ampere flowing for one second) 
is said to have a capacity of one farad. The microfarad 
(too^^oFo f3.rad) is used in practice, as the farad is too 
large for ordinary use. 

Experiment 147. Connect a telephone condenser of 2.5 microfarads 
in series with a switch, a direct current source of potential of about 
60 volts, and a galvanometer, Fig. 325. Close 

the switch and notice the "kick" of the gal- 1 ^;^J C^/" 

vanometer. Disconnect from the source of 

potential the charged condenser, and connect _ 

its terminals directly to the galvanometer. fig. 325. -Experiment 

Notice the " kick " of the galvanometer as the showing charging of Con- 
condenser discharges. When the condenser denser with Direct Cur- 
is once charged, it may be said to have an rent, 
infinite resistance to direct currents of constant potential. 

Effect of Capacity in an Alternating Current Circuit. — 

Consider an alternating e. m. f., as in Fig. 326, to be con- 
nected to the terminals of a condenser, and assume that 
its wave shape will be as shown. Assume that the con- 
denser possesses no initial charge and that it is connected 




CAPACITY E.M.F 



256 EXPERIMENTAL ELECTRICITY 

to the circuit when the e. m. f. is passing through the zero 
point. Since there is no initial charge in the condenser to 
resist the flow of energy into it, the rate of flow of current 
into it at this same instant of time will be a maximum. The 

effect is somewhat 

analogous to that 

LINE CURRENT produced by steam 

^ discharging into a 

MOTION 1 ., 

vacuum, where its ve- 
locity may be 30,000 
feet per minute. As 
the flow of current 

Fig. 326. — Capacity and Current Relations. • - ^t_ j 

^ ^ ^ mto the condenser 

continues, its potential increases, and this, acting like a 
counter e. m. f., tends to cut down the current flow, the 
charging current falling to zero when the alternating po- 
tential has reached its maximum value. When the line 
potential starts to fall, the condenser potential, because 
greater for the instant, tends to discharge back into the 
circuit, and the discharge increases in magnitude as the 
alternating potential falls, and reaches its maximum value 
when there is zero initial alternating potential to resist it. 
From this discussion it may be noticed that there is a con- 
tinual charging and discharging of the condenser, the con- 
denser at one instant taking energy from the circuit, and 
at the next giving it back. If we represent the cycle of 
e. m. f. in Fig. 326, by 360°, dividing it up into four 
elements of 90°, and consider that the direction of motion 
is to the right, it will be observed that the capacity 
e. m. f. lags the line current (crossing zero) 90°. The 
relation between the line current and the e. m. f. due to 
capacity in a circuit may be represented by means of 
their effective values in the form of vectors, as in Fig. 338. 



PRINCIPLES OF ALTERNATING CURRENTS 257 

The mathematical expression for the e. m. f. due to capacity 
in a circuit may be obtained in the following manner : 

Let C be the capacity in a circuit. 

Let e be the potentiiil of the capacity when charged. 

Let /"be the frequency of the circuit. 

Then Ce, or capacity times potential, according to our 
definition, will represent the charge expressed in coulombs 
which the condenser accumulates. In a cycle of alternating 
current, it is evident that if the condenser is connected 
to an alternating current circuit it will be charged and 
discharged 4 times per cycle f. The average rate of 
charge could then be expressed as 4/. This rate of 
charge, however, must be converted from average rate to 
maximum rate. 

Maximum = — average. 
2 

Maximum rate of charge and discharge = ^—^ = 2 irf, 

2 

Multiplying the rate by the charge Ce, we obtain 
I=2irfCe, ~=2irfC, 1= , 



211/0 

where /= the current passing through circuit. / and ^ 
are both effective values. 

Problem. What is the capacity reactance x of 50 microfarads of 
capacity in an alternating current circuit of 60 cycles ? 

= 53.2 ohms. 



2 tt/C 2 X 3.14 X 60 X .00005 

Capacity reactance, -, is expressed in ohms. If re- 

2 irfC 

sistance is present in the circuit, its value in ohms must be 

combined vectorially with the capacity reactance in ohms 




258 EXPERIMENTAL ELECTRICITY 

to obtain the equivalent effect. For instance, a capacity 

reactance of 4 ohms and an ohmic resistance effect of 
3 ohms would result in an equivalent 
effect of 5 ohms. The capacity reactance 
is plotted graphically as in Fig. 338, where 
it is 90° out of phase with the ohmic 

FIG. 327. -Relation of resistance effect. Squaring both sides 
Sides in Right-angle of the right-angled triangle, and extract- 
Tnangie. -^^ ^j^^ squarc root of their sum, we have 

the value for the hypothenuse of 5 ohms. 

Thus, 3^= 9 

4^- 16 

Total, 25 

sum = 25 ; square root of sum = 5. 

Series Parallel Combinations of Condensers. — Condensers 
when placed in series. Fig. 328, tend to __ |_ 

decrease, in inverse proportion, the total * 1 — ' * |j^ — • 

capacity of the system. The effect is just fig. 328. — Condens- 
opposite to resistance circuits where two ^^^ ^^ Senes. 

resistances placed in series have a combined resistance 
equal to their sum. 

Experiment 148. Place a 2. 5 -microfarad condenser in series with 
a projecting galvanometer and a 60-volt source of direct current 
potential, and note the deflection of the gah'anometer on closing the 
switch. Place two 2.5-microfarad teleplione condensers in series in 
a similar manner, and notice that the deflection of tlie galvanometer 
on closing the switch is half as great as before. Discharge the con- 
densers in both cases through the galvanometer, and notice that the 
deflection reduces to \ in the latter case. 

When condensers are placed in parallel, Fig. 329, their 
capacity increases proportionally in a similar manner 



PRINCIPLES OF ALTERNATING CURRENTS 259 

to series resistances. Two equal resistances placed in 
parallel have an equivalent resistance equal to \ of one 
of the resistances. Two equal resistances placed in series 
have a combined resistance equal to their sum. Two 
condensers of equal capacity placed in 



E 



& 



parallel have a combined capacity when 
operating equal to their sum. Two con- 
densers of equal capacity placed in series 
have a combined capacity equal to % of fig. 329. -Condensers 

, ^ ^ ^ . in Parallel. 

the value of one of them. The capacity 
of cables used for underground distribution is therefore 
directly proportional to their length, and their charging 
current will likewise increase as their length increases. It 
will be shown later that capacity reactance and inductance 
in an alternating current system may be made to balance 
each other. In practical operation the capacity of the sys- 
tem is fixed, as the length of the cables when once installed 
is fixed. Inductance is added to the system or taken from 
it by varying the field rheostats on the converters, motor 
generators, generators, etc., until a balance occurs and unity 
power factor exists. For a series combination of condensers 
their equivalent sum can be obtained mathematically as 
follows : 

_ I 

c^ c^ c^ 
Self-induction. — As has been said, self-induction is 
present in a direct current circuit as well as in an alternat- 
ing current circuit, and it affects the circuit whenever there 
is any change in the current flow or the flux of the circuit. 
When the flux in a circuit changes, owing to a change 
in the current flow, the flux cuts the turns of wire that 
produce the field, causing an e. m. f. to be induced in 



26o EXPERIMENTAL ELECTRICITY 

the winding. The magnitude of the induced e. m. f., 
termed the e. m. f. of self-induction, depends upon two 
things : first, the quantity L termed the coefficient of 
self-induction, or the number of lines of force linked with 
the circuit per absolute unit of current ; and second, the 

quantity f — ), which expresses the rate of change of the 

current. It can readily be seen that the quantity L is 
dependent upon the original strength of the field or the 

number of ampere turns, whereas the quantity f — ] is 

dependent upon the current flowing in the circuit, and 
the frequency with which it rises and falls from maximum 
to zero, and zero to maximum. L is usually expressed in 
henry s. The henry may be defined as that constant by 
which the time rate of change in a circuit must be multi- 
plied in order to give the e. m. f . induced in that circuit. 
One henry exists in a circuit when a current varying one 
ampere per second produces one volt of e. m. f . in that 
circuit. In practice the henry is so large that a smaller 
unit, the millihenry, or j-qVo" ^^ ^ henry, is used. The 
e. m. f. of self-induction may then be expressed by the 
formula 



.dt. 

The negative sign here is due to the fact that this 
e. m. f.. Eg, is a counter e. m. f. 

Example. Being given a current in a circuit which changes from 
o to loo amperes in .005 second, and assuming that the counter e. m. f., 
E^, is 15 volts, what would be the coefficient of self-induction of that 
circuit ? 

If the current varies from o to 100 amperes in .005 second, the 

quantity — would become = 20,000 ; or the time rate of variation 

^ ^ dt .005 



PRINCIPLES OF ALTERNATING CURRENTS 26 1 

of the current would be 20,000 amperes in a second. The equation 
would then become, substituting 1 5 for Eg, and 20,000 for — , 
15 =Z (20,000), 



The following interesting example is given in Foster's 
Handbook, page 65. A coil of wire of 150 turns is carry- 
ing 2 amperes and producing a field of 200,000 lines of 
force. If it takes the current i second to die out when 
the circuit is opened, what will be the e. m. f. of self- 
induction induced in the circuit ? If 2 amperes produces 
a field of 200,000 lines of force, i ampere will produce 
100,000 lines of force. If it takes the current i second 
to die out, then each turn would cut 100,000 lines of force 
in that time, and 1 50 turns would be equivalent to i turn 
cutting 15,000,000 lines of force in a second. As i volt 
equals 100,000,000 fines of force cut per second, an e. m. f. 
^^ iVoV^VoVo"' °^ -^5 ^^^^' would be generated, or .15 
henry. 

From the above example and from the definition of the 
coefficient of self-induction, Z, it will be observed that 
the value of L may be expressed in terms of the flux, 
the turns, the current, and 10^ power, as follows : 10^ 
= 100,000,000. 

^ _ flux X number of turns (^N 



current x io« /V2 x 10^ 

/V2 = maximum amperes, where /= effective amperes. 

In an alternating current circuit the e. m. f. of self- 
induction, E, is expressed in terms of the frequency, /, 
the current, /, the coefficient of self-induction, Z, and 
the constant, 2 tt, in the following manner : 
Z = 2 irfLL 



262 EXPERIMENTAL ELECTRICITY 

This equation may be obtained by considering a coil 
of wire in which an alternating current is passing, the 
coil having n turns, inclosing a flux ^, the current having 
a frequency/. Then the average e. m. f., ^ avg., which 
would be induced in this coil, would be 

As the flux would rise and fall from zero four times in 
a cycle, so four times the flux, times the turns, times 
the frequency or cycles per second, would give the total 
lines of force cut in one second. Dividing this by lo^ 
(100,000,000 lines of force cut in one second equals one 
volt) gives the average e. m. f. generated. To find the 
effective voltage, the voltage usually used, it is necessary 
first to find the maximum e. m. f., E max., from the average 
value. It may be remarked incidentally that the equation 
above for E avg. is \\\^ fiindamental equation for alternators, 
transformers, induction motors, and practically all alternat- 
ing current apparatus. 

From page 257 we have the maximum value E max. 

equal to —E^yg. and the effective e. m. f., Ef= -=-^. 

2 V2 

it max. = — ii avg. ; —=^ — ^ — =1.57; 
222 

^ E max. /- 
Ef=—-=-; V2=i.4i. 

The average value of an a. c.-e. m. f. is .633 times as 
great as its maximum value, and the maximum value is 
1. 4 1 times as large as the effective value. Alternating 
current voltmeters indicate effective values. 



PRINCIPLES OF ALTERNATING CURRENTS 263 

Simplifying, 

Uf^N ^ TT ^ 2 nTf(bN \ 

V 10^ 2 10^ y' 

r, 2 Ivf^N 

h max. = — '^ — ; 



10= 

as 2 = V2 X V2- 

V2 X V2 7r/(^7\^ I -\/^iTf^N' 
then £^.= o -^ X— ^:^ ^^^i—- 



10^ V2 10 

a/2 7r/(|)i\^ 



From page 261 

L = -^ — - or (f>A^= Z/V2 X 108. 
/V2 10^ 

Substituting for (f)N m above formulae, 

^,- = ^ = 2 irfLI. 

Ef= 2 irfLL 

In this equation Ef is expressed in volts, f in cycles 
per second, L in henrys, and / in effective amperes. 

This e. m. f. of self-induction, Ef, in an alternating cur- 
rent circuit tends to lead the a. c. current in the circuit 
by 90°, Fig. 330. In considering the total 
e. m. f. required to send an alternating cur- 
rent through an inductive circuit, we find 

that the e. m. f. is made up of two parts, 

, ^ , . Fig. 330, — Relation 

the e. m. r. necessary to overcome the resist- of Resistance and 

ance of the circuit, and the counter e. m. f. inductive e. m. f. 

of self-induction induced in the circuit. In considering the 

operation of a shunt motor, it may be remembered that a 

counter e. m. f. was also present and that this e. m. f. was 



I R 



264 EXPERIMENTAL ELECTRICITY 

subtracted from the line e. m. f. in order to determine the 
e. m. f. forcing current through the armature resistance, or 

= /, The counter e. m. f . of a motor is, therefore, 1 80° 

r 

apart from its Hne e. m. f., but the e. m. f. of self-induction 

, ^ must be added vectorially. Thus in Fig. 

^ I ,,. '} 331 is shown a line e. m. f. of 100 volts, a 

he2c^ — 80 — 5>i counter e. m. f. of 20 volts, and a resulting 

Fig, 331. — Line E.M.F. direct Current voltage in a 

and Counter E. M.F. , , x o^ li. 

motor armature of 80 volts. 
In Fig-. 332 is shown a line e. m. f. of 100^. ^ . 

^ ^ ^ Fig. 332. — Resist- 

volts, an inductive e. m. f. of 20 volts, and an ance and induc- 
e. m. f. A to overcome resistance of *^^^ e.m.f.'s. 

1002=202+^2^ 

^2_ jqq2 _ 2o2, 
^ = Vl002 — 202 = 97.9. 

To show that the e. m. f. of self-induction is 90° ahead 
of the current, consider the alternating current curve 
ABCDE, Fig. 333. It may be remembered that in order 

to have an e. m. f. induced in a 
winding it is necessary that the flux 
should change. In the curve, Fig. 
333, there are two points, B and 
Dy where the current is neither in- 
creasing or decreasing. At these 
two instants of time, theoretically. 
Fig. 333. -Alternating Current the Current must be stationary and 

E.M.F. Curve. ^r, n ^ -u ^ ^- u 

the flux must be stationary, result- 
ing in zero e. m. f . of self-induction being generated. When 
the current begins to fall from B, the flux being in phase 
with it, the e. m. f. of self-induction assumes a greater and a 
greater value as the curve becomes steeper, until when the 





PRINCIPLES OF ALTERNATING CURRENTS 265 

current is passing through zero, the flux is changing at its 
maximum rate, the e. m. f. of self-induction being a maxi- 
mum, as shown in Fie:. 

' <^ ^^j^SELF 1N3UCT10N 

334. As a single lobe 
of an alternating current 
wave is represented by 
180°, it will be noticed 
that, considering the mo- 
tion to be toward the right, 

the curve of self-induction ^^^- 334.-Relation of Current and Indue- 

tion E. AI.F.'s. 

passes through the zero 

line 90° before the current has crossed the zero line. As 
the e. m. f. which overcomes the resistance of a circuit is 
in phase with its current, it may be stated that the e. m. f. 
of self-induction is 90° ahead of the a. c. current in the 
circuit. These values may be represented in terms of 
effective values instead of instantaneous values, vectors 
being used, as in Fig. 332, the inductive e. m. f. leading 
by 90° the resistance e. m. f. 

Experiment 149. Place a coil of a large number of turns but of low 
resistance (Long Tom) in series with a i6-candle-power lamp and a 
direct current source of potential. Notice that the lamp will light to 
practically full intensity, but that it is a little sluggish in so doing. 
Substitute then an alternating current source of supply of 60 cycles of 
about the same potential for the direct current potential, and notice that 
the illumination of the lamp is almost reduced to zero. Introduce an 
iron core in coil and extinguish lamp. Vary it in and out. If 25 cycles 
is used, the low tension windings of two 20-light Type H Transformers 
placed in series will form a suitable inductance. 

Experiment 150. Vary frequency with same set-up and notice that 
inductive reactance increases with an increase in frequency. A small 
rotary converter operated from the direct current side with a field rheo- 
stat in circuit forms a suitable method of varying the frequency. 

Experiment 151. Form a series circuit of condensers, inductance, 
and a i6-candle-power lamp. Vary inductance until a maximum of 



266 EXPERIMENTAL ELECTRICITY 

candle power is reached. In this case the capacity reactance neutraHzes 
the inductive reactance. About 40 microfarads of capacity is a suitable 
amount of capacity to use. 

Experiment 152. Measure with a projecting voltmeter thee.m.f.'s 
across the individual units of the previous set-up and show vector rela- 
tion. Notice that sum of e. m. f.'s is greater than line e. m. f. Plotting 
the 90° relation of inductive reactance, resistance, and capacity reactance, 
the resultant may be obtained. 

Experiment 153. Form a multiple circuit of inductance, resistance, 
and capacity. Potential across elements of circuit will be the same. 
Measure individual current values with projecting ammeter. Notice that 
line current is less than sum of individual currents. Calculate admit- 
tance (see page 272) and plot vectors. When current passing through 
inductance is alone indicated, add capacity in parallel and notice that 
ammeter reading becomes smaller. 

Experiment 154. With first inductance and then capacity alone in 
an alternating current circuit, calculate coefficient of self-induction L 
and capacity C. It is necessary to know the frequency of the circuit, 
the current passing through the device, and the potential across it. 

Vectors. — It is convenient in considering alternating 

current circuits to represent the current and pressure 

^ — ^ values by means of vectors. A vector is an 

MOTION arrow, Fig. 335, which may be considered to 

rotate in a counter-clockwise direction, whose 

length represents to convenient scale the 

Fig. 335.— Vector, effective value of electromotive force or 

current, and whose direction with some other <:^ 

X MOTION 

vector represents the angular phase displace- 
ment between the two. Thus, in Fig. 336, is 
shown graphically the relation between the 
e. m. f. of self-induction, 2 irfLI, and the cur- 
rent / in the circuit. The angle of lag Q of i 
the current /behind the e. m. f. is in this case Fig. 336. —Vector 
90°. Figure 337 shows an e. m. f. in phase ^^'^^'°" °^ ^": 

^ fc) JJ/ r ductance and 

with the current. This condition holds in Current. 



PRINCIPLES OF ALTERNATING CURRENTS 267 



^ — ^—i- 



t 



H 



in Fig. 361 



FIG. 337. — E. M.F. 
and Current in 
Phase. 



3TI0N 
I 



2 7rfc 



an alternating current circuit containing only resistance. 

Figure 338 illustrates a circuit containing capacity in which 

the current leads the capacity reactance by 

90°. A three-phase circuit in which the 

e. m.f.'s are 120° from each other may be 

shown as in Fig. 339. This is termed a 

Y connection. A delta connection is shown 
Usually the delta connection 
is shown as an equilateral triangle. The re- 
lation between the primary e. m. f., the sec- 
ondary e. m. f., and the flux of a transformer 
maybe represented as in Fig. 340, the primary 
and secondary e. m. f.'s being 180° apart. A 
more lucid exposition of the subject of vectors 
applied to transformers may be found in the 

Fig. 338. — Vector chapter on transformers. 

Relation of Cur- 
rent and Capac- Where resistance is present alone in an 
ity Reactance, alternating current circuit the e. m. f. repre- 
senting the difference of potential across 
the terminals of the resistance is in phase 
with the current, the 
conditions being similar 
to a direct current cir- 
cuit, Fig. 341. Where 
inductance and resist- 
ance are both present in 
the circuit, the e. m. f. required to send a 
current through the circuit is made up of 
FIG. 340. - Relation ^'^^ Components, Fig. 342. In this dia- 
of Transformer gram AB is the e. m. f. vector RI neces- 
E.M. F.'s gg^^y |-Q overcome the resistance of the cir- 

cuit, A Bis the current vector /in phase with the resistance 
e. m. f., CB is the inductive e. m. f. vector, 2 tt/LI, at 90° 




Fig. 339. — Y Con- 
nector of Three- 
phase Circuit. 



268 EXPERIMENTAL ELECTRICITY 

to the resistance e. m. f., and AC is the resultant e. m. f . 

£ = /VR^-\-{2'Tr/Lf equal to the vector sum of the other 

two e. m. f.'s in the circuit. The value AC is obtained by 

1^ E ^ taking the square root of the sum of the 

i ^i squares of the other two sides, this being 

1 I the familiar geometrical relation for the hy- 

' ' pothenuse of a right-anp-led trianscle in terms 

Fig. 341. — Resist- ^. . , ^_ ^ ^, .^^ 

anceaionein Ai- ^^ 1^^ Other two sides. Thus, if One Side is 
ternating Cur- 3, the Other side 4, the hy pothenuse will 
rent Circuit. ^^ ^ ^^^ ^^^^^ ^^ j,.^^ ^^^^ represents the 

amount the current in the circuit / lags behind the result- 
ing e. m. f., or the line e. m.f., E. It will be noticed that 
the extent of this phase displacement 
depends upon the relative magnitude 
of the vectors AB and BC If AB 
is large and CB small, that is, if the 
inductance of the circuit is small 
compared with its resistance, the 
phase displacement will be small, fig. 342.— Vector Relation of 

If the vector BChQ large compared R^^^^tance plus inductance. 

with AB, the angle will be large, The latter condition is 
met with in a Type I, Thomson Recording Wattmeter, 
where two fields are produced by two windings, one a 
highly inductive winding, the resultant field being rotating. 

Another thing may be noticed about Fig. 342, and that 
is that the sum of the two vectors AB-\-BC is greater than 
AC. A straight line is the shortest distance between two 
points. In other words, the sum of the individual e. m. f.'s. 
in a circuit may be greater than the line e. m. f., AC 

Trigonometric Expressions. — In trigonometry we have a 
means of expressing the relations of the sides of a right- 
angled triangle in terms of its central angle. This rela- 
tion is expressed by what are known as sines, cosines, tan- 





PRINCIPLES OF ALTERNATING CURRENTS 269 

gents, these being used more frequently than other trigono- 
metric expressions. In Fig. 343 the sine of the angle Q 
is equal to BC divided by A C, the cosine by c 

AB/A C, the tangent by BC/AB. The ratios 
just given are definite for any acute angle. 
Tables are prepared which give the values ^ vl^ 

of sines, cosines, tangents, etc., for different ^^^- 343-— Trigo- 
nometric Rela- 

angleS. tions in a Right 

In alternating current work one is con- Triangle, 
cerned mainly with angles of 30°, 45°, and 60°. In an 
equilateral triangle such as is formed by the vectors of a A 
three-phase circuit the angles between the vectors are 60°, 
and the sides are equal. If we drop a perpendicular from 
one of the angles, we form a right triangle, as in Fig. 343, 
whose sides we can designate by the numerals i, 2, V^, 
for the square of i is i, the square of the V3 is 3, the 
square of 2 is 4; 4 = i + 3- In this right triangle one has 
a 30° and a 60° angle. The sine, cosine, and tangent are 
therefore as follows : 

sin30°=^=-:^.5=cos6o°; 

AC 2 

cos 30° = 41= — = '^^^ = sin 60° ; 
AC 2 

tan 60° = ^ = ^= V3= 1.74. 
Cn I 

From the above it is evident that the sine of one angle is 
equal to the cosine of its complementary angle. Thus 
sine of 30° equals cosine of 60°, subtracting 30° from 90°. 

Where a 45° right triangle is used, two sides may be 



2/0 EXPERIMENTAL ELECTRICITY 

designated by the numerals i, and the third side by the 

V2, in which case the sine and cosine are both equal to ~= 

V2' 

It is evident that by the use of trigonometry in an alter- 
nating current circuit, if the angle of phase displacement 
of line e. m. f . and current is known and the line e. m. f is also 
known, the inductive e. m. f. and the resistance e. m. f. may 
be determined, if they alone are present. For instance, 
in Fig. 342, 

. /, BC 2 irfLI 2 irfL 
sm c/ = — - = ^ = — ; 

^C /V^2_^(2 7r/Z)2 V7?^ + (2 7r/Z)2 

r, AB RI R 

cos 



^C /Vi?2_p(2 7r/Z)2 -^R'^ + {2irfLY 

/) BC 2 irfLI 2 irfL rL 

tan V = = — ^ = — - — — 2 Trf— ■ 

AB RI R -^ R 

The ratio — is sometimes called the time constant of 

a circuit. Numerically it is equal to the time necessary 
for the current to rise to ^ of its maximum value. For a 
given R, the greater L is, the longer will be the time nec- 
essary to reach this value or the larger will be the time 
constant. 

The above equations may be rewritten in the form, 

BC=ACx sin6>; 
AB = AC xcosO; 
BC=ABxt2in6. 

Alternating Current Circuits containing Resistance, Induc- 
tance, and Capacity. — It has been shown in previous para- 
graphs that in an alternating current circuit containing 
resistance, inductance, and capacity, the e. m. f.'s across the 



PRINCIPLES OF ALTERNATING CURRENTS 2/1 



terminals of each of these elements in a series circuit is 

/ 



equal to RI, 2 irfLI, and 



The graphical represen- 



2 n-/ L I 



2 77 / C 



R I 



Vector Rela- 
tion of E. M. F.'s. 



2 7rfC 

tation of these elements in a series circuit is illustrated in 
Fig. 344, where the resistance vector ^ — ^ 
^(T leads the capacity reactance vector 
AD, and lags the inductive vector AB. g 
Since the effect of ca- 
pacity in the circuit is 
opposite to the effect of 
inductance, these two 
vectors may be sub- 
tracted from each other, 
or added vectorially. fig. 344 
Fig. 345. —Vector Whichever is the greater 

Relation of E. (dominates the phase displacement in the 
circuit. If the capacity reactance is greater 
than the inductive reactance, Fig. 345, the current in the 
circuit will lead ; if, on the other hand, the 
inductive reactance is the greater, Fig. 346, 
the current will be lagging. Instead of plotting 
vectors in terms of e. m. f., their value in ohms 
may be used, and this 
will eliminate the quan- 
tity /. Thus, when the 2^/^ 
inductive reactance vec- 

FiG.346.-Efrect ^^^ ^^^ ^^^ Capacity re- 
opposite to Fig. actance vector have been 
345- added, the parallelo- 

gram may be completed as in Fig, 
347, the resulting line AB, or the fig. 347. — Ohmic Relation 
diagonal of the parallelogram, rep- ^^ ^^^^^^^"^^ ^^^ R^^^^^^^^- 
resenting the algebraic sum of the three vectors and the 




272 EXPERIMENTAL ELECTRICITY 

impedance of the circuit Analytically this process may 
be represented as follows : 

AD=-^. AF=2iTfL, AC = R, 
2'Tr/C 

AE=2'rTfL--^, AB^^R'^^U-rrfL--^-), 
2 7rfC ^ \ 2 7rfCj 

AB is usually represented by the letter Z. 



Z = \R'^^[2'KfL "^-^ 

\ 2 IT ft . 

When 2 irfL — — ^, the quantity (2 tt/Z ^— ) 

reduces to zero, in which case V^ = i?, or Z = 7?, the 
conditions, so far as ohmic effect is concerned, being simi- 
lar to those in a direct current circuit. This condition is 
termed resonance. When a resonant condition does not 
exist, the e. m. f. current relations can be shown by the 
relation -c- -p 



z J.. . / .r r^ 



v^^n^-^^-^rJ 



The quantity is expressed in ohms and is termed 

2 17/0 

the capacity reactance of the circuit. The quantity 2 irfL 
is expressed in ohms and is termed the inductive reactance 

of the circuit, and the quantity ( 2 irfL -— ) is also 

expressed in ohms and is termed the reactance of the cir- 
cuit. It is sometimes represented by X. The recipro- 
cal of the impedance — = Fis termed the admittance. In 

order to combine reactances, Fig. 348, which are in par- 
allel, it is necessary first to determine their admittances 



PRINCIPLES OF ALTERNATING CURRENTS 273 



wvww- 



3 



ijMiMsiifiiyi. 



Fig. 348. — Parallel 
Reactances. 



and their phase relations, plotting them vectorially, Fig. 

349. In this case we plot current values, while in a series 
circuit potential values are used, for in 
the case of admittances I-^ — E Y^, and 
I^ — E Fg, etc., where /and E are the cur- 
rent and e. m. f. values and F^, Fg are the 
admittances. The polygon will then as- 
sume the form of Fig. 349. For specific 
examples, see Sheldon's Alternating 
Currents, page 89, and Hay's Alternating Currents, page 21. 

Power of an Alternating Current Circuit. — The power of 
an alternating current single-phase circuit, irrespective of 

the phase relation of 
^ current and electro- 
motive force in the cir- 
cuit, can be obtained 
from a consideration 
of the instantaneous 
values of pressure and 
current in a similar manner to a direct current circuit. 
For example, if at a given instant of time we multiply to- 
gether the corresponding val- 
ues of current and pressure, 
we obtain a true value in 
watts for the power at that 
particular instant. From this 
fact it will be noticed that 
the power in an alternating 
current circuit is continually 
changing, rising from zero to 
a maximum, and decreasing to 
zero again, as shown in Fig. 

350. The power curve will have positive values when both 




Fig. 349. — Vector Relation of Admittances. 




Fig 350. —Power in a Single-phase 
Circuit. 



274 EXPERIMENTAL ELECTRICITY 

current and pressure values are positive, + x + = +, and 
positive values when both current and pressure values are 
negative, — x — = +. If current and pressure values are 
positive and negative, the power will be negative, + x — = — . 
When the power is positive, the generator is supplying power 
to the line; and when the power is negative, the line is re- 
turning power to the generator. Thus, in a single-phase sys- 
tem, there are times when power is being returned to the 
system. In a three-phase system this condition does not oc- 
cur. The amount of power returned in a single-phase circuit 
depends upon the phase difference of the current and pres- 
sure. If the angle of lag be as much as 90°, the power returned 
(negative loop) will be equal to the power delivered (posi- 
tive loop). The power absorbed during a quarter period 
is returned to the circuit during the next quarter period. 
This condition of a 90° lag is impossible, however, in any 
single-phase circuit containing resistance, as it requires some 
power to overcome this resistance. The ordinary Thom- 
son Recording Wattmeter will indicate the true power 
in a single-phase circuit because the magnetic strength 
of current and pressure coils will vary with the instanta- 
neous values of current and pressure in the circuit and 
the resulting torque of armature will vary with the instan- 
taneous power in the circuit. A more compact meter, 
such as the G. E. Type I, utilizing the 

L E ^ lagging effect of self-induction, is used, 

^i ^te H^ however, for alternating current single- 

1^ J J phase power measurements. With this 

meter, to be described later, the weight 
Fig. 351. — E.M.F. and q£ ^]-^g moving element is reduced to a 

Current in Phase. . . , . . , 

mmimum, thus obviatmg as much as 
possible the loss due to friction. When the e. m. f . and the 
current are in phase, as in a circuit containing only resist- 




PRINCIPLES OF ALTERNATING CURRENTS 275 

ance, Fig. 351, the product of the two vectors will give the 
true power in the circuit. In a circuit in which the e. m. f. 
AB and the current Zf^' differ in 
phase, Fig. 352, by the angled, 
the projection of the vector AB 
on AC or A B (obtained by drop- 
ping a perpendicular from Z> to ^46^) !=£ 1 ->j 

must first be obtained before the fjg. 3S2.-E.M.F. and Current 
product AB x AC will yield the o^t of Phase. 

power in watts in the circuit. Considering that ABD is a 
right-angled triangle, the relation of AB to AD may be 
expressed in terms of the angle 6, 

AB/AD = cos e, 
or AB = AB X cos 0. 

Power Factor. — As has already been stated, the power of 
an alternating circuit containing inductance and capacity 
cannot be obtained by simply multiplying together the 
effective values of current and pressure, as might be done 
in the case of a direct current circuit. 

The quantity cos 0, derived above, is termed the J?ower 
factor of the circuit. When the angle 6 = 0, or when AD 
and ^ 6^ coincide, Fig 353, the power factor is imitj/, coso° 

-^^^ = I, for the cosine of Fig. 353 is ex- 

^ ^ ^0 ^0 pressed AB/AD; as AB will equal 

Fig. 353. — Unity Power ^Z^ when part of the same straight 
^^^^°^- line AB/AD = 1. In practice power 

factors of alternating current circuits, if supplying arc 
lamps, are about 85%. Where a large induction motor 
load is on the system, the power factor may be as low as 
60%. Where motor generators are used employing syn- 
chronous motors, the power factor is greatly improved. 



2/6 EXPERIMENTAL ELECTRICITY 

This is one of the chief advantages urged for the use of 
the synchronous motor. 

To obtain the power factor of any circuit 
such as an inductance, an ammeter, a volt- 
meter, and a wattmeter may be used as in 
Fig- 354- The ammeter and current coil of 
the wattmeter are placed in series with the 
inductance and a source of alternating po- 
^^mentai Method tential, and the voltmeter and potential 
of measuring winding of the wattmeter shunt the device. 
Power Factor. Calling the wattmeter reading P, the volt- 
meter reading E, and the ammeter reading /, the power 
factor may be obtained as follows : 
P = ETcose', 
P = E X I X power factor; 
P 




Ex I 



— power factor. 



Experiment 155. Measure the power factor of an alternating cur- 
rent arc lamp, using a wattmeter, ammeter, and voltmeter. Do not meas- 
ure the arc voltage, but rather the voltage across the whole lamp. 
The power factor should be about .85. 

When operating alternating current machinery, it is 
always desirable to operate as near unity power factor as 
possible, otherwise an additional current will flow in the 
circuit, unnecessarily heating the windings. It is evident 
that where the power factor is other than unity a certain 
amount of energy representing the true power in the cir- 
cuit is dissipated, whereas other energy, not dissipated 
and termed wattless energy, is pumped back and forth in 
the circuit. This energy is stored in the magnetic field at 
one instant in the circuit and returned to the generator at 
the next instant. 



PRINCIPLES OF ALTERNATING CURRENTS 277 

Power Measurements in a Three-phase Circuit. — On a 

three-phase circuit two wattmeters may be used to measure 

power, the current coil of each watt- V~\T 

meter being placed in one phase, Fig. 

355. The potential coils of each 

wattmeter are connected from the 



phase wire, in which the current coil \tt 

is located, to the common third phase ^^^ 355. -Power Measure- 
wire. With this connection the sum ment in a Three-phase 

of the two wattmeter readings equals Circuit. 
the totalpower in the circuit. For power factors below -5 
the reading of one wattmeter will be negative, the total 
power being the arithmetical difference of the two readings. 
The power factor of a balanced three-phase circuit may be 
obtained by means of the tangent formula 

tan^=V3-^^7rfr^;- 

In this formula W^ and W^ are the wattmeter readings. 
Having determined the tangent of the phase angle, the 
angle itself is determined from trigonometric tables. The 
cosine of this angle is the power factor. Curves may be 
plotted which will yield the power factor directly when 
values of W^ and W^ are known. A complete mathemati- 
cal exposition of this method may be found in Frederick 
Bedell's Direct and Alternating Current Testing, page 
232. A curve is given on page 232 showing the rela- 
tions of W^ and W^. The points on this curve are de- 
termined from the relation 

W^ ^ cos (6 + 30°) 
IV^ cos (6 - 30°)* 

The larger reading is W^ and will always be positive; W^ 
is the smaller reading and may be positive or negative. It 



278 



EXPERIMENTAL ELECTRICITY 




TermincI Soau 



Fig. 356. — G. E. Single-phase Induction Wattmeter. 




^ Seal Pin 
•Snipping Device <Spnnff and Cap ' 
Paris of Ttiom'Sor? High Torque Induction fleeter Type I . 

Fig. 357. — G. E. Single-phase Induction Wattmetter. 



PRINCIPLES OF ALTERNATING CURRENTS 279 

is well in measuring the power on a polyphase system to 
consider the circuit as made up from a number of single- 
phase circuits and to determine the sum of the energy 
consumed in each individual phase. 

Single-phase Integrating Wattmeter. — The single-phase 
wattmeter, Figs. 356, 357, 
358, has two windings, a ^W) 
series winding and a po- _ 

tential winding wound ^^S 
upon parts of the same 
magnetic circuit. The 

shunt winding is highly fig. 358. — jewel Bearing of Wattmeter. 

inductive and consists of a large number of turns of fine wire 
wound upon a laminated iron core. The circuit is highly 



®^ 





Fig. 359. — Induction Wattmeter. 

inductive and its current lags almost 90° behind its im- 
pressed e. m. f. 

The series winding is practically non-inductive and con- 
sists of a few turns of heavy wire. On a non-inductive 

load the current in this winding 

will be in phase with the impressed 

e. m. f. The two magnetic fields 

iNDocEoiiN sh6rt'V ^ o^ shuut aud series windins; will 

^ E.M.F. SHUNT COIL ^ 



M2 



SHUNT COIL 



CIRCUITING COIL 



FIG. 3<ic.-Vec,or Relation of therefore be 90° apart, Fig. 360. 

Single-phase Meter E. M. F.'s. Both of these magnetic fields act- 



280 EXPERIMENTAL ELECTRICITY 

ing on an aluminum disc produce a resultant field which is 
rotating, and this causes the disc to rotate. Eddy currents 
produced in the disc react and cause rotation, as the result- 
ant field will vary with the product of the impressed e. m. f. 
and the current in the circuit, the torque of the disc will 
likewise vary. Magnets perform the same function in this 
meter as in the direct current Thomson Recording Watt- 
meter. In order that the meter may read power correctly, 
it is necessary that some compensation should be made for 
copper and iron losses in the shunt circuit which keeps the 
phase difference between the flux of the shunt circuit and 
the series circuit from being exactly 90°. One or two turns 
of wire are therefore placed on the projection of the 
pole of the shunt coil. These produce an induced field, 
which, acting with the shunt field, corrects for losses. 
This correction will vary if the power factor of the circuit 
varies to any extent, and adjustments should therefore be 
made for the power factor upon which the meter is intended 
to operate. Figure 358 shows a special form of spring 
bearing used in the Type I, G. E. meter. For complete 
descriptions of other types of alternating current meters 
see the comprehensive report of the meter committee of 
N. E. L. A., 1909. For friction curves of the Type I meter, 
see page 228. 

Experiment 156. Calibrate a Type I single-phase meter on various 
inductive loads, and correct for power factor. 

Polyphase Integrating Wattmeters. — Polyphase inte- 
grating wattmeters of the induction type consist of single- 
phase elements assembled on the same shaft. On a three- 
phase circuit when operating at unity power factor one 
winding of the meter will tend to lead, and the other to lag 
by 30°, the combined effect being neutral. In Fig. 361 let 



PRINCIPLES OF ALTERNATING CURRENTS 28 1 




Fig. 361. — Relation of 
Current and Poten- 
tial Vectors in a 
Polyphase Meter. 



^(7 equal the potential winding of one element and BC the 

potential winding of the other element, the corresponding 

series windings being in line wires A and 

B. The current in phase A is displaced 

30° from its potential AC, and in phase 

B the displacement is the same, although 

in the opposite direction. Each element 

on a three-phase meter at unity power 

factor load operates at about Z6%y the 

cosine of 30°. 

When the current in a three-phase cir- 
cuit lags 30°, the condition is similar to that shown in Fig. 
362, the meter operating at a power factor of ^6(^o- The 
current in phase B lags its respective 
potential 60°, while the current in 
phase A is in phase with its poten- 
tial. One single-phase element oper- 
ates at 50% power factor, while the 
other element operates at unity power 
factor. One element will therefore 
tend to run twice as fast as the other 
element. When the three-phase cir- 
cuit lags 60°, or is operating at a 
power factor of 50 %, the current in phase B lags its poten- 
tial 90°, operating at o power factor ; while the current in 
phase A lags its potential 30°, operating at ^6 % power fac- 
tor. Under these conditions one eleftient has stopped and 
the other element is doing all of the work. When the 
current lags 90°, or zero power factor, one element has 
a power factor of — 50 % and the other element has a 
power factor of -f-50%. The meter element will not 
move under that condition, as one element will try to 
operate at one half speed in one direction while the 




Fig. 362. — Polyphase Meter 
Vectors Current lagging 30°. 



282 EXPERIMENTAL ELECTRICITY 

Other element tends to operate at one half speed in the 
opposite direction. 

QUESTIONS 

1. Define the henry and the farad, and explain liow capacity and 
inductance affect an alternating current circuit. 

2. What is a vector and how may it be used ? 

3. Upon Avhat principle does the G. E. Type I, single-phase meter 
operate ? 

4. Show mathematically that a polyphase meter with two potential 
and two current elements indicates the total power in the circuit. 

5. How is rotation produced in an alternating current wattmeter ? 

6. Why does an indicating wattmeter indicate tlie true power 
in a circuit ? 

7. What is meant by the term }'eso7iaiice f 

8. Why is the sine of an angle equal to the cosine of its comple- 
mentary angle ? 

9. Are capacity and inductance present in a direct current circuit, 
and if so, when do they affect the circuit } 

10. What method would you employ to obtain a set of instantaneous 
current voltage and power curves from an alternating current generator .^ 



CHAPTER XV 
THE ALTERNATING CURRENT TRANSFORMER 

The alternating current transformer is a simple device, 
consisting of two coils, or a multiple of two, wound upon an 
iron core. An alternating e. m. f. is applied to the terminals 
of one coil, termed the primary, the potential being either 
raised or lowered across the secondary terminals, in pro- 
portion to the relative number of turns in both windings. 
By such an arrangement it is possible either to raise 
the potential of a few volts to several thousand volts or 
to lower the potential of several thousand volts to a few 
hundred volts. As the transformer is such a flexible 
piece of apparatus, it is possible to utilize it in transmitting 
high potential power, using a comparatively small wire, 
thus reducing to a minimum the amount of metal required. 
The transformer is undoubtedly the principal element in 
the alternating current system. By means of this device 
it is possible to reach over a much 
greater territory with alternating current 
distribution than can be done with direct 
current distribution, where the copper 
loss would be prohibitive. 

Theory of the Transformer. — Suppose 



tsTOD 



that alternating potential is placed upon Fig 363. — Experimen- 
the terminal of a coil of wire which is f ^'^^^ ^^ '^'^^'- 

former. 

wound upon an iron core as in Fig. 363. 
An alternating current will traverse the winding and set 

283 



284 



EXPERIMENTAL ELECTRICITY 



^ 



SECONDARY 



Fig. 364. — Simple Trans- 
former. 



up an alternating magnetic field in the iron core. This 
alternating flux will generate in the primary winding an 
alternating e. m. f ., termed the e. m. f . of self-induction. 
Owing to the magnitude of this e. m. f., which lags 90° 
behind the exciting e. m. f., the excit- 
ing current that passes through the 
winding will be small. This so-called 
magnetizing current is quite small 
in commercial transformers. It can 
be realized that this is a necessity 
of design, as the primary windings 
of transformers are connected con- 
tinuously in service, and the energy 
which is lost in magnetizing the trans- 
former must therefore be reduced to 
a minimum. Suppose that a second winding is placed 
upon the same core as in Fig. 364. This may be termed 
the secondary winding. Suppose also that this winding 
is open-circuited at its terminals or is not 
connected to a load. As the alternating 
flux of the primary winding cuts the sec- 
ondary winding, it generates an e. m. f. in 
this winding. This induced e. m. f. is 90° 
behind the flux of the primary winding, 
or 180° behind the primary e. m. f. It 
may therefore be termed a counter e. m. f. 
The relation between the primary e. m. f., 
the flux, and the secondary e. m. f . is shown 
graphically in Figs. 365, 366. If both 
primary and secondary coils have the same 
number of turns and there are no losses, the primary 
e. m. f . and the secondary e. m. f. would be equal, as there 
would be the same flux interlinked with each turn and 



^ PRIMARY 
E.M.F. 



SECONDARY 
E.M.F. 



Fig. 365. — Relation of 
Transformer Vectors. 



THE ALTERNATING CURRENT TRANSFORMER 



;85 



PRIMARY 
E.M.F 




SECONDARY 
CURRENT 



SECONDARY 
E.M.F. 



the rate of motion of the flux would be the same for both 
windings. Suppose that a resistance is connected across 
the secondary winding of a i to i transformer as in 
Figs. 364, 367, allowing 
a current to circulate 
through the secondary 
winding. If this resist- 
ance be a lamp bank, 
the lamps will light if 
the voltage relations are 
proper, although the 
current which will pass 

through the lamp circuit fi^. 366. -Relation of Instantaneous E. M. F.'s. 

may be greater than the 

current which formerly passed through the primary wind- 
ing. The question arises as to how this current can pass 
through the secondary winding in this 
case with a i to i transformer, if it is 
greater than formerly passed through 
the primary winding. It passes because 
the current circulating through the sec- 
ondary winding produces a flux which 
slightly counteracts 
the e. m. f. of self- 
induction of the primary winding. Since 
the ampere turns of the primary and 
the secondary winding are opposite, 
more current will enter the primary 
winding. If an ammeter, Fig. 368, be 
placed in series with the primary circuit nect'ed 
and one also in series with the secondary ^0^^"^^^- 
circuit, it will be noticed that when the load is placed upon 
the secondary circuit that the primary current increases. If 





Fig. 367. — Load Con- 
nected to Transformer. 



Mm^ 







D D 




Fig. 368. 



— Load Con- 
to I : I Trans- 



286 EXPERIMENTAL ELECTRICITY 

a I to 2 transformer is used, it will also be noticed that as 
the secondary current increases with increase in load the 
primary current will increase twice as much. So soon as 
this inductive effect is decreased, it allows more current 
to pass through the primary circuit. If the secondary cir- 
cuit be completely short-circuited through zero resistance, 
a maximum of current willpass through the primary wind- 
ing, this current being limited in magnitude only by the 
resistance of the primary winding. The fuses in the 
circuit will consequently blow. Most commercial lighting 
transformers used on the distributing system of lighting 
circuits are provided with four windings, two primary 
and two secondary. By this arrangement it is possible 
to produce various ratios of transformation. It sometimes 
happens in connecting up the secondary windings to 
produce a three-wire distribution, the primaries being fed 
from a single-phase circuit, that the terminals of the 
primary are not connected in the proper sequence, the 
end of one coil to the beginning of the other coil. When 
such a transformer connection is placed in operation, 
the primary fuses blow because the self-induction of the 
primary-winding has been neutralized by having the 
ampere turns of the primary windings opposed. It is 
obvious, therefore, that the transformer is perfectly auto- 
matic in its action and requires practically no supervision. 
As the current is being transformed, the transformer 
undergoes certain losses in energy termed resistance loss, 
eddy current loss, and hysteresis loss. As these losses 
have been treated of previously in connection with other 
apparatus, no further mention need be made of them here 
except to state that combined they represent a small total 
and thus allow high efficiency. For a 400-kw. Westing- 
house transformer operating 6200 volts to 170 volts, that 



THE ALTERNATING CURRENT TRANSFORMER 287 

is, having a ratio of 37 to i, the losses and efficiency are 
as follows : 

LOSS 

Full load iron loss, 6,330 watts 
Full load copper loss, 4,356 watts 

Total, 



100% load, 
75 % load, 
50% load, 
25% load, 

100% load, 
75 % load, 
50% load, 
25 % load. 

These are used in a single phase 
of a double delta 6-phase transformer 
as installed by the large lighting com- 
panies. 

For details as to the mathematical 
calculation of these losses the reader 
is referred to the author's Electric 
Railways, Vol. II, Page 219. During 
the past few years much improvement 
has been made in the manner of 
designing transformers, as shown in 
Figs. 369, 370, the weight of the 
transformers per kilowatt being much 
reduced by obtaining better trans- 
former iron and improvinsr the mae:- 

... r o o YlG. 369. — Modern method 

netic circuits. of building Transformers. 





10,686 watts 


EFFICIENCY 


97-8% 


(400-kw.) 


96.5% 




94.0 % 




91.0% 




9775% 


(800-kw., three such used 


97-25 % 


for a 2000-kw. rotary 


96.50% 


converter) 


93.50% 






288 



EXPERIMENTAL ELECTRICITY 



There is one other loss which has not been discussed, 
namely, the leakage of flux in the transformer. This 

depends upon the de- 
sign of the transformer. 
M/CC7 S/7/e/c/s WllllllKl/l/ Seconc/c7ry In some cases, where 

Fr/marc/ the primary and second- 
ary windings are sepa- 
rated, this cross flux 
may be used to secure 
automatic regulation 
for producing constant 
current or constant 
power. Figure 378 




O/'/Duct 



O/'/'C/van/ye/s 



Fig. 370. — Cross section of Fig. 369. 

shows a subway transformer, and 
Fig. 369 shows a modern shell type 
transformer. 

Experiment 157. Connect the secondary 
winding of a constant potential transformer to 
a suitable source of potential, say the low ten- 
sion side of a I kw. Type 
H-G. E. Co. transformer 
to a 120-volt alternating 
current source of potential. 
Place an ammeter in the 
circuit, and notice the low 
magnetizing current. 

Experiment 158. With 
the same set-up as in the 
Fig. 372. — Show- previous experiment connect a lamp board to the other 
ing Principle of secondary winding, using one of the secondaries for 
Transformer. the exciting circuit. Be sure that the high tension 





Fig. 371. — Queen & Co. Ex- 
perimental Transformer. 



THE ALTERNATING CURRENT TRANSFORMER 289 



windings are insulated or disconnected, as a dangerous high potential 
sufficient to kill will be generated in the high tension windings. Keep 
decreasing the resistance in the sec- 
ondary circuit by turning on lamps, ^___^ U ' I 
and notice that the primary current ^ III J ^ iCC I ^ 1 

keeps increasing. \^ L 

Experiment 1 59. Take a coil of No. p^^_ ^^^_ Reluctance of Transformer. 
16 wire wound upon an iron core and ^jj. q^d shown, 

pass an alternating current through 

the coil of about 10 amperes, having a suitable resistance in the circuit, 
Figs. 371, 372. Do not leave the coil on the circuit long, as it will 
heat up very quickly. Have another coil, ar- 
ranged as in Fig. 372, consisting of a large 
number of turns into which the smaller coil 



^ VWV^ 




3) 









Fig. 374. — Study of 
Transformer. 



will pass. To the larger coil connect a 16- 
candle-power lamp. Bring the smaller coil 
excited with alternating current into the larger 
coil, and notice that the lamp will light. 
Experiment 160. Take apart an old Type 
H transformer G. E. Co. and connect a lamp to the high tension wind- 
ing on one coil, and excite through a suitable resistance the low tension 
winding on the other coil. The ratio of these 
windings is 20 to i for some transformers and rpr 

10 to I for other transformers. Take a 20 to i | 

ratio. Bring one pair of coils near the other ' 

so that the ends of their cores touch, and notice ^^^* 
that the lamp will light. Move coils a very 



=ic: 



375- — Varying Re- 
luctance of Transformer. 



i=-<D-^ 



small distance apart, Fig. 373, and notice the great diminution in candle 
power. When cores are in contact place a number of stampings around 
the outside of the coils connecting the two 
free ends of the cores, and notice that the 
candle power of the lamp will increase. 
Explain this phenomenon. 

Experiment 161. Place a wattmeter in 
circuit with the low tension winding of a 10 
to I transformer, the other winding being open circuited, and notice the 
small wattmeter reading. This value represents the iron loss in the 
former. Use only one winding, as in Fig. 374, and vary resistance in 
circuit. Also vary magnetic circuit, as in Fig. 375. 



imn 



Fig. 376, — Measuring Copper 
Loss of Transformer. 



290 



EXPERIMENTAL ELECTRICITY 



Experiment 162. Place the wattmeter in the circuit as in the last 
experiment, and arrange an additional adjustable series resistance in the 
primary circuit. Place a short-circuiting strip 
across the secondary terminals, Fig. 376. We 
are considering the two low tension windings 
of a Type H transformer, one as primary and 
the other as secondary. Vary the resistance in 



1 



Fig. 377. — Efficiency. 



the circuit and take a series of readings of the wattmeter. If an ammeter 

is also in circuit, the corresponding readings may 

be taken. These wattmeter readings will give 

the copper loss for this winding for various loads. 

Short-circuiting the free secondary winding 

eliminates the iron loss in the transformer, as it 

neutralizes its self-induction. 

Experiment 163. Measure the efficiency of a 
transformer, Fig. 377, by placing a wattmeter in 
the primary circuit and a wattmeter in the sec- 
ondary circuit, and by placing a variable load 
upon the secondary. A projecting wattmeter 
may be used for this purpose, arranging a short- 
circuiting switch for the current terminals in each circuit and a double- 
throw switch for potential. 

Types of Transformers. — Two types of transformers are 
commonly used by modern lighting companies, air-cooled 




Fig. 378. — Manhole 
Transformer. 




Fig. 379. — Manhole with Transformer installed. 



THE ALTERNATING CURRENT TRANSFORMER 291 



Porcelain Bush/. 



■ng 



Cambric Covered Cable. 



— Compound 



fnsu/aiion Tube 

inter ieaf Terminai- 
Barrel 



Sleeve 

Wiped Joint 



transformers and oil-cooled transformers. The air-cooled 
transformers are used in station operation, and the oil- 
cooled transformers are used in the distribution system. 
The small oil-cooled transformers are used on pole lines, in 
manholes, as in 
Figs. 378, 379, 
380, or on the 
customer's prem- 
ises. Subway 
transformers 
must be water- 
tight, requiring 
special arrange- 
ments of leads. 
Where the trans- 
mission voltage 
is high, such as 
80,000 volts, it is 
necessary to in- 
sulate the transformer windings again with oil, and to keep 
the oil cool by means of pipes in it through which cold 
water circulates. With the ordinary air-cooled transformer, 
such as is used in central stations, air passes up through 
the transformer windings, this air being circulated by a 
motor-driven blower. Some companies use shunt motors 
for this purpose, but an induction motor is preferable, as a 
very large amount of dirt is drawn in by the blower from 
the floor of the station, this dirt depositing upon the com- 
mutators of the motors and causing the insulation between 
the windings to burn out. Service transformers are pro- 
vided with two primary and two secondary coils, although 
it is becoming the practice to divide each secondary coil 
into two parts so that a balanced three-wire system can be 




—Paper Insulated 
Lead Covered Cable 



Fig. 380. — Waterproof Lugs for Subway Transformer. 



292 



EXPERIMENTAL ELECTRICITY 



obtained, part of each secondary winding being in each leg 
of the circuit. 



000000 
1 2 3 4 5 



1 2 3 

vcLTsI 272 136 



5610 




NAAAAAAAAAAAAAAA/WV] [^A/WWVW^AMAAAAA^/| 
7 8 9 10 



VIEW OF 

BOTTOM OF 

TRANSFORMER 



^ 



^ 



10 



H.T. LEADS 

n n n n n n 




SECONDARY 
TERMINALS 



SECONDARV 



i35i"j rc5==03'i TcgziziQii' 



'J 



1 



I 9 10 I I 9 10 



'secondary 
delta reversed 



Fig. 381. — Station Transformer Connections, Six-phase. 

Station Transformers. — The practice of central station 
companies at the present day in this country is to go in 
extensively for the use of three-phase transmission at 25 
cycles, using a double delta connection of transformer 



THE ALTERNATING CURRENT TRANSFORMER 293 



secondaries, 
Figs. 381, 382, 
producing six- 
phase, which is 
led to the shp 
rings of the con- 
verter. The 
converter has a 
larger capacity 
for six-phase 
than for three- 
phase, its capac- 
ity increasing 
with the num- 
ber of phases. 
Where 60-cycle 
transmission is 
used, the motor 
generator system 
is preferred, as 
it has been found 
practically im- 
possible to keep 
60-cycle convert- 
ers from flash- 
ing over their 
commutators. 
As the substa- 
tions are some- 
times located at 
a considerable 
distance from 
the main power 
house, resulting 




VIEW SHOWING L.T. TERMINALS 
BOTTOM VIEW 



C 



^ 




/ V 


(0 

















) 


1 


2 


3 


4 


5 


6 






VIEW SHOWING H.T. TERMINALS 

HrGH TENSION TERMINALS AT TOP OF TRANSFORMER 
LOW TENSION TERM IfJALS AT BOTTOM OF TRANSFORMER 



I9.3 



4.6 



\AAaaaaaaa/ 



/W^AAA/V^AWVVV 



7 8 y 10 

Fig. 382. — Station Transformer Connections, Six-phase. 



294 EXPERIMENTAL ELECTRICITY 

in a considerable fall in potential on the alternating current 
feeders, it is necessary to provide the transformers with a 
number of taps in order to have a standard size which will 
be applicable to any part of the system. The diagrams, 
Fig. 382, show this method for a 6290-volt system, where 
the transformers have a ratio of 37 to i, transforming the 
voltage at anywhere from 6450 to 5610 volts to 170 volts. 
In Fig. 382 it will be noticed that primary taps i, 2, 3, 4, 
5, 6 connect to the primary windings and produce between 
respective pairs a distribution of potential of 272, 136, 
5610, 272, 170, volts. The secondary taps, 7, 8, 9, 10, 
coming from the bottom of the transformer, are connected 
in double delta, as shown in Fig. 382, the secondary taps 
being invariable. The location of the transformer primary 
and secondary leads as they leave the transformer are 
shown in Fig. 381. For a 200-kw. Westinghouse trans- 
former and a 400-kw. transformer the taps and voltage 
relations are given in the following table, where the 
figures in the first column represent the voltages at vari- 
ous points on the system, and those in the second column 
give the taps to use to produce 170 volts alternating 
current. 

In selecting a set of taps, the average voltage at the point 
should be taken. It is possible to raise or lower this 170 
volts alternating current feeding the converters by means 
of the induction regulators. The converters with compen- 
sating poles or the converters with interpoles dispense 
with induction regulators. The 200-kw. transformer is 
arranged to give 186 volts instead of 170 volts in the 
following table. These transformers are air cooled. Their 
frames are permanently grounded so as to mimimize danger 
in case the high tension windings should become grounded 
to the shell. 



THE ALTERNATING CURRENT TRANSFORMER 295 

Connections for 200-Kw. Transformer 

With H. T. Voltage of 6450 connect to Terminals i &6 
With H. T. Voltage of 6310 connect to Terminals 2&6 
With H. T. Voltage of 6045 connect to Terminals i &5 
With H. T, Voltage of 6200 connect to Terminals 3 &6 
With H. T. Voltage of 5910 connect to Terminals 2 «&: 5 
With H. T. Voltage of 5610 connect to Terminals 3 &4 
With H. T. Voltage of 5720 connect to Terminals 2&4 
With H. T. Voltage of 5800 connect to Terminals 3 & 5 

Connections for 400-Kw. Transformer 

With H. T. Voltage of 6450 connect to Terminals i &6 
With H> T. Voltage of 6300 connect to Terminals 2 & 6 
With H. T. Voltage of 6195 connect to Terminals i &5 
With H. T. Voltage of 6045 connect to Terminals 2&5 
With H. T. Voltage of 5985 connect to Terminals i &4 
With H. T. Voltage of 5835 connect to Terminals 2&4 
With H. T. Voltage of 5600 connect to Terminals 3 &4 

With high tension connections as shown each half of low tension 
of 200-Kw. Transformer will dehver 186 volts 
of 400-Kw. Transformer will deliver 170 volts 

Generally taps i and 5 are used with a 400-k\v. trans- 
former, using a primary voltage of 6290 and a secondary 
of 170 volts: 6290/170 = ratio of 37/1. These trans- 
formers may be used as auto transformers if occasion 
requires. For instance, a voltage of 5600 volts could be 
fed into the transformer at taps 3 and 4, and 6450 volts 
could be taken out at i and 6. The induction regulators 
used in connection with these transformers have a high 
efficiency also, as, for example, 

100% load 93.6% 

75 % load 92.6 

50% load 90.5 

25% load 84.3 



296 



EXPERIMENTAL ELECTRICITY 



Subway Transformers. — Subway transformers, Figs. 
378, 379, 380, are somewhat similar in construction to the 
regular transformers, except that they have to be circular 
in shape and water-tight. The entrance holes for the leads 
in the case of the transformer are provided with closely 
fitting bushings, Fig. 380, through which pass snug-fitting 
lugs, into which the leads of the transformer are sweated. 
The cover of the transformer is fastened in place with 
bolts passing through a rubber gasket. In all underground 
work it is necessary to take the same precautions that 
would be required in operating a submarine system, since 
the cables and transformers are frequently covered with 
water. 

Ratios of Type H Transformers. — The small Type H 
lighting transformer may be used in so many different ways 
that a few details concerning its various 
possible ratios are pertinent. Many of 
these combinations are possible with 
other small transformers. The primary 
and secondary winding of these trans- 
formers have a ratio of either 10 to i 
or 20 to I ; the ratio of 10 to i will here 
be considered. In the low tension wind- 
ings the transformer 
may be used as i :2, i : J, i : i, as sho^m 
in Figs. 383, 384, 385. By using one 
primary and one secondary, a ratio of 
10: I may be produced ; by using one pri- 
mary and two secondaries in series, a ratio 
of 20 : I may be produced. By using two 
primaries in series and one secondary, a 
ratio of 5 : I may result. By using two ^^^^f^^^ 
primaries in series and two secondaries in 



MVV^VV^V^A/VV'VY^/V^VVWVW/^V^V^ 



Fig. 383.- Type H 
Transformers i : 2 Ratio. 



WVVVVW^\M/V] 



D 



6 

Type H 
Transformer i : | 
Ratio. 




THE ALTERNATING CURRENT TRANSFORMER 297 

series, a ratio of 10:1 will result, or with the same primary- 
connection but with the secondaries in parallel, a ratio of 
20 : I results. By placing both prima- 
ries in parallel and both secondaries in T T 
parallel, a ratio of 10:1 results, and by wwwJ 
placing the secondaries in series and Fig. 385.— Ratio i;i. 
the primaries in parallel, a ratio of 20 : i results. Usually 
both primary and secondary coils are used, a potential of 
2240 volts being transformed to 1 12-224. Slightly higher 
voltages are frequently used. 

For details as to Scott's two- to three-phase connections, 
transformer types, structural features, transformer oils, heat- 
ing of transformers, regulation, and losses, the reader is 
referred to the author's Electric Railways, Vol. II, where 
substation operation is discussed in detail. 

Experiment 164. Connect a low tension winding of a Type H trans- 
former to a ii6-volt source of alternating current potential, and to the 
secondary terminals connect a ii6-volt lamp showing i : i ratio. 

Experiment 165. Connect both low tension terminals of the same 
transformer in series to a source of alternating current potential, and 
connect the lamp across one of the coils in shunt showing i : \ ratio. 

Note. — In experimenting with the low tension tei^minals 
of transformers, be sure to insulate the high tension terminals, 
P n as they will possess high potential which 

^ 1 will be dangerous. 

W _ -„-„._.„..J Experiment 166. Connect a i6-candle-power 

lamp in series with one coil of a Type H trans- 
former, and an alternating cur- q q 
rent source of potential, and j (j j) .^j. a ^y. T 
Fig. 386. - Neutral- short-circuit the other winding, ^ig. 387. — Trans- 
izing Self-induction and the lamp will light, Fig. 386. former Secondaries 
of Transformer. Experiment 167. Connect in Series, 
both secondary windings of a transformer in series with a i6-candle- 
power lamp and an alternating current source of potential, reversing 






298 EXPERIMENTAL ELECTRICITY 

the connections of one of the secondaries, as in Fig. 3S8. Notice that 
the lamp will light. 

Experiment 168. Make the same setup, 

y ^ I — — 1 but reverse another one of the transformer 

j^ connections, Fig. 387, and notice that the 



Fig. 388. — Transformer lamp will not light. 

Secondaries connected in j^^ instantaneous Current and pres- 

Opposition. ^ 

sure curves of a transformer may be 
obtained by means of the contact maker and the tele- 
phone balancing circuit described on page 244. For the 
complete mathematical treatment of the transformer, the 
reader is referred to Steinmetz's Alternating Current Phe- 
nomena, pages 193 to 236, and for details as to design, 
to the Alternating Current Transformer, by Baum, and 
to Transformer Design, by Adams. 

QUESTIONS 

1. What is a transformer ? How is it constructed, and how does 
it operate } 

2. Show vector relation of e. m.f.'s in a transformer. 

3. In w^iat manner may the self-induction of the secondaries of a 
transformer be neutrahzed ? 

4. What ratios of transformation are possible with a Type H-G. E. 
Co.'s Transformer? 

5. Name the principal methods of cooling transformers. 

6. What method could be employed to determine the beginning and 
the ends of the secondary coils of a transformer so as to know the proper 
series connection? 

7. Draw a vector diagram to show how six-phase transformation 
may be obtained from three-phase transformers. 

8. Draw diagram of circuits to show when there would be attrac- 
tion and repulsion between transformer windings. 

9. How may the core loss, the copper loss, and the efficiency of a 
transformer be measured ? 

10. Is the efficiency of a transformer high or low, compared with 
other pieces of electrical apparatus ? 



CHAPTER XVI 



THE INDUCTION MOTOR 



Theory. — Two phenomena occur when two coils carry- 
ing a. c. current are brought near each other, one of the 
coils having a resistance shunted 
across it, and the other coil being 
the exciting circuit. One of these 
phenomena is a transformation of 
pressures, the other is a mechanical 
reaction or repulsion between the 
coils. In the ordinary transformer 
the coils are fixed in position and 
the potential transformation feature 
is utilized, while in the induction 
motor. Fig. 389, the coils are mov- 
able, the mechanical reaction be- 
tween them being utilized in addition 
to the potential transformation fea- 
ture; indeed, the latter does not 
play quite so important a part as 
the former. The operation of the 
induction motor may best be illus- 
trated by considering the squirrel- 
cage induction motor. This motor 
is very simple in construction, con- 
sisting of a circular core contain- 
ing slots, in which are placed the 

magnetizing conductors or stator winding. This winding is 
symmetrical and is supplied usually with out-of-phase cur- 

299 




FiG. 389. — Induction 
Motor. 



^•K. 



300 EXPERIMENTAL ELECTRICITY 

rents, two or three-phase, producing a rotating field. The 
moving element, or rotor, consists of a number of lamina- 
tions assembled on a shaft containing holes or slots in the 
periphery through which are passed copper rods, these rods 
being short-circuited upon each other at each end. When 
the motor is stalled so that rotation cannot occur, the alter- 
nating current passes into the stator winding, inducing low 
voltage currents of large capacity in the short-circuited 
secondary winding of the rotor. This feature is similar to 
the operation of ordinary step-down transformers. If the 
brake is removed, repulsion between the rotor and the 
stator causes the rotor to turn and as the out-of-phase 
currents produce a shifting or rotating field — the north 
pole traveling around the rotor — the repulsion of the 
rotor is continued. The rotor speed when the machine is 
unloaded is sHghtly less than the speed of the rotating 
field, this difference in speed being termed slip. When 
a load is placed upon the motor, this slip increases, and 
the slip in turn increases the induced current in the rotor 
winding and thus increases the torque. This process will 
continue up to a certain point, beyond which the limit of 
stability of the machine is exceeded, and the machine 
will come quickly to rest. 

Experiment 169. Dismantle a small induction motor, and excite 
the three-phase winding from a three-phase circuit, placing a resist- 

ance in series with each leg of the circuit, 

Fig. 390, so as to limit the current input to 
about 4 amperes. Support the motor on 
blocks in a vertical position, resting on the 
lower bearing as is shown in the horizontal 
,AAAAAA^ — ■■ projection. Have convenient a hollow 




Fig. 390. — Experimental Study copper sphere, Fig. 391, such as is used as 

of Induction Motor. a float in reservoirs. This copper sphere 

should be either suspended on a string or supported on a \" rod so that 

it can rotate between the fingers when held loosely. Excite the three- 



THE INDUCTION MOTOR 30I 



i 



phase circuit of the motor, and suspend the copper sphere inside of the 
frame. Rotation will immediately occur, the sphere revolving at a high 
speed. Change over any two of the three leads of 
the motor and notice that the direction of rotation 
of the sphere changes. The copper sphere in this 
experiment corresponds in an elementary way to 
the rotor of the induction motor, out-of-phase cur- 
rents being induced in the sphere and producing a y\g. 391!^ Copper 
rotating field which follows the rotating field of the Sphere, 

stator winding. 

Experiment 170. Use only one of the phases excited as before and, in 
place of the copper sphere, bring inside of the motor frame a coil of fine 

wire of many turns with an iron core so 
that the single-phase alternating current 
in the rotor winding will induce an e. m. f. 
in the stator winding. Measure this e. m. 
f. with a voltmeter, a projecting voltmeter 
if a lantern is being used. Fig. 392. This 
simple experiment illustrates the trans- 
former action taking place in the machine. 
Fig. 392. — Transformer Feature of Experiment 171. Excite one of th e 
Induction Motor Shown. ^^^^^^ ^f ^ three-phase machine with 

direct current fi-om a ii6-volt source of potential. Fig. 393, a suitable 
resistance being placed in circuit. Move around in the winding the 
north pole of a compass 
needle, and notice that sepa- 
rate poles exist — say four 
poles, two north and two 
south. Excite another phase 

in the same manner and 

, . ^1 , r 1 Fig. ^gq. — Poles of Induction Motor Shown, 

notice that four more poles are ^^^ 

produced, then excite the third phase and notice that four more poles 
are produced. Each of the phases produces the same number of poles, 
these being symmetrically distributed over the periphery of the stator. 
Thus, at a given instant of time, when supplied with alternating cur- 
rent, four main poles will be produced, each one made up of sets of 
three coils, and each of these having the same polarity as in Fig. 
394. At the next instant of time — looking clockwise — the first one of 
all of the sets of south poles will become a north pole, and the 




r^)) 


—^\l\hhN\l 


x^m -*- 




^■116 VOLTS 









302 EXPERIMENTAL ELECTRICITY 

first one of each of the sets of north poles will become south, as in 
Fig. 394. In other words, the relative separate poles exist, but there 

^,.N .^ is a continual progression around the stator. In 

^^V^-T^s-^r^s. \ a three-phase circuit the phases are 120 degrees 

Kj/'/^'S-^nIZ|^n.n \ "s apart, a complete rotation of the field depending 

/ / / f f^^'^^^^^ \ \ \ upon the number of pairs of poles. Thus, a four- 

1 "^ 'tl Wv tfl^l ) poleinductionmotor operated from a two-pole gen- 

\ \ N^s/N^.^_.s''y^/iv/ / erator will rotate only half as fast as the generator. 

V^stl^-s-^'VV operation of an Induction Motor. — 

^'~~^~' An induction motor is essentially a con- 

^'of S!es~ofi?duction stant specd machine. Its speed may be 
Motor with Aiternat- varied by varying the number of poles, a 
ing Current. method now being used on small ma- 

chines made by the General Electric Company and the 
Westinghouse Electric Manufacturing Company. To set 

up an induction motor and to operate 

it is only necessary (for small machines) ~ 

to connect the phase terminals directly 

to the source of supply. With a two- 

phase machine, Fig. 395, two of the fig. 395.— Two-phase 

1 1 i. J X • Induction Motor. 

phases may be connected, formmg a 
two-phase three-wire system wired up to the transformers, 
the transformers being connected in the same manner. 
Care should be taken to fuse the neutral wire heavier than 
either side of the system, as the load on the neutral leg is 
heavier than on either outside legs. 

In starting machines of five horse power and more, it is 
customary to have a starting resistance either internally 
or externally connected. This high resistance gives a high 
starting torque, and the elimination of this resistance at 
the higher speeds gives a high efficiency. 

A three-phase induction motor may be operated from a 
single-phase circuit. Fig. 396, by means of various phase- 
splitting devices. A condenser compensator, consisting 




THE INDUCTION MOTOR 



303 



of a combination of capacity and inductance or a combina- 
tion of choke and resistance coils, produces the desired 
effect. 



Pr/ma/^y C/rcu/t. 



mPr/mary 3*y/tc/J 
ancf f'u.se 3oif. 




Fig. 396. — Three-phase Induction Motor operated from Single-phase Circuit. 

In certain classes of work the induction motor is prefer- 
able to other types of motor. In central station work for 
the driving of blowers, Fig. 397, they are used almost 
exclusively, although there are some cases in which shunt 



304 EXPERIMENTAL ELECTRICITY 

motors are used for this purpose. Induction motors are 
preferable to shunt motors for blower service, because the 
blowers are continually drawing air in through the motor, 




Fig. 397. — Induction Motor operating a Blower. 

which deposits the dirt on the windings of the motor and 
also on the blades of the blower. It has been remarked 
that "a blower is the dirtiest piece of apparatus to clean 
in existence." Where a shunt motor is employed, this dirt 
accumulates on the commutator, and the commutator burns 
out, necessitating frequent repairs. The small induction 
motor is used extensively in the outlying districts of light- 
ing companies when alternating current is employed for 
distribution. The two-phase motor is used mostly for this 
work. Its advantage lies in the facts that a combined 
lighting and power system may be operated from the same 



THE INDUCTION MOTOR 305 

set of mains, and that where there is a possibihty of having 
lighting customers alone, only one phase of the two-phase 
system is employed. With power circuits two transformers 
are installed on the pole line near the customer's premises, 
and the low tension sides of the step-down transformer are 
placed in series, producing a three-wire system of 240 
volts between each phase, and 240 x V2 = 338 volts across 
the outside legs of the circuit. 

Changing Direction of Rotation. — The direction of rota- 
tion of a three-phase induction motor may be changed by 
changing over any two of the phases. This, as has been 
said, causes the rotating field to rotate in the opposite 
direction, the drag on the conductors of the rotor pulling it 
around. 

In starting an induction motor of large capacity, 100 kw. 
for instance, it is common to have two sets of taps 
leading from a step-down transformer to a double-throw 
switch. One set of taps supplies normal voltage, and 
the other type supplies \ ox \ normal voltage. At start- 
ing, the switch is thrown on the low voltage side, and after 
the machine has attained speed, the switch is closed in the 
opposite direction, putting normal voltage on the machine. 

Experiment 172. Change over terminals of induction motor, caus- 
ing the direction of rotation to change. 

Care of Induction Motors. — Owing to the simphcity of 
the induction motor, its lack of brushes, brush holders, 
commutators, and slip rings, operators are likely to neglect 
this piece of apparatus. Dirt and oil are Hkely to collect 
in the windings, in which event they become short-circuited 
and burn out. This is all the more likely to occur, as in- 
duction motors are usually mounted in places somewhat 
inaccessible, owing to their simplicity. Induction motors 
should be cleaned and inspected at regular intervals, like 



3o6 EXPERIMENTAL ELECTRICITY 

other station apparatus, and should receive their propor- 
tionate amount of care. 



QUESTIONS 

1. Compare the operating features of an induction motor and a 
transformer. 

2. How is rotation caused in an induction motor? 

3. Why does changing over any two pair of leads of a three-phase 
induction motor cause the direction of rotation to change ? 

4. Considering the induction motor as similar to a transformer in 
magnetic features, how would you measure the losses of the motor ? 

5. Why is it necessary to use a starting device for large capacity 
induction motors? 

6. Why is an induction motor easier to maintain than a direct cur- 
rent motor? 

7. Why is it necessary to fuse the neutral leg of an induction motor, 
two-phase three-wire, heavier than either outside wire ? 

8. Why is it that a \ horse-power induction motor will not be injured 
if you place a load upon it sufficient to bring it to a standstill ? 



CHAPTER XVII 

THE ROTARY CONVERTER 

The rotary converter is a machine used for converting 
direct current into alternating current, or alternating cur- 
rent into direct current. It is most frequently used to 
convert alternating current into direct current. The con- 
verter possesses an armature surrounded by a field wind- 
ing. The armature possesses a single winding, having a 
commutator connected at one end, and slip rings at the 
other end. A shunt motor may readily be made into a 
rotary converter by mounting sHp rings either over part 
of the commutator, if the commutator is long, or mounting 
slip rings on the other side of the armature from the com- 
mutator end, connecting the slip rings to the winding at 
equidistant points. For a three-phase rotary the taps 
should be connected at three points 120° apart. Convert- 
ers may be single-phase, three-phase, six-phase, twelve- 
phase, etc. Three-phase and six-phase machines are used 
most frequently, the latter often being preferred, owing to 
its increased capacity. An interesting feature in connec- 
tion with a converter is that, while operating, converting 
alternating to direct current, it can also be belted to a load 
and made to deliver mechanical power at the same time. 

E. M. F. Relations. — Owing to the fact that there is but 
one winding on the ordinary converter and that also there 
is but one field winding, barring booster converters, the 
direct current voltage and the alternating bear a definite 
relation to each other. It is possible to vary this ratio 
slightly by varying the field excitation, sufficient, however, 

307 



3o8 EXPERIMENTAL ELECTRICITY 

for load regulation. Varying the field excitation changes 
the power factor. In the new type of interpole con- 
verters and booster converters the ratio can be changed 
over quite a large range. This feature will be discussed 
later. The coefficients by which the voltage between the 
direct current brushes must be multiplied in order to obtain 
the effective alternating current voltage between adjacent 
slip rings or rings of the same phase are as follows : 

2 rings 0.707 

3 rings 0.612 

4 rings 0.500 
6 rings 0.354 

Consider a two-pole converter armature, Fig. 398, containing 
two slip rings connected to taps 180° apart. Between these 

two sHp rings there will be 
^t^\ \ P^^d^^s^i 3, single phase 

e. m. f ., Fig. 398, whose maxi- 

FIG. 398. -Converter Armature Connec- ^^^^ ^^^^^^ ^^'^^ ^^^'^^ ^^'^^ 

tions. of the e. in. f. existing be- 

tween the direct current brushes^ assuming a two-pole 
machine. The relation between the maximum value of 
an alternating e. m. f. and its effective value is given by 
the expression i/V2- The maximum value of E.^,, which is 
the same as the direct e. m. f. divided by the square root of 
2 or E^/^2, equals the effective value, or 

E=EJ-^^. 

Instead of dividing E^ by V2, the voltage E^ may be 
multiplied by the reciprocal of this quantity, or 1/^2, or.707. 
Em may be considered in this case as equal to the direct 
e. m. f. and E as corresponding to the single-phase alter- 
nating e. m. f. This method of reasoning may be applied 





THE ROTARY CONVERTER 309 

to a three-phase e. m. f., except that in this case the figure 
assumes an equilateral triangle, and the relation between 
one of the sides and the diameter is taken as 1/V3. 

Problem. Deduce factor 0.612 for a three-phase machine. 

Experiment 173. Take a three-phase converter and measure with an 
alternating current voltmeter the voltage between successive slip rings 
and between the direct current brushes. Operate the converter from 
the direct current side with a field rheostat in the circuit. Vary the 
field excitation which will change speed of converter or its frequency 
and notice inappreciable change in the alternating e. m. f. 

As such features of the rotary converter as magnetic 
circuits and tests for trouble are similar to those of a shunt 
motor, it is unnecessary to describe them here ; instead, 
we may discuss operating features and recent develop- 
ments in converter manufacture. 

Methods of Starting Converters. — Three methods are in 
common use for starting converters, although this number 
could be increased by modification of the primary methods. 
The converter may be started from the direct current side 
as a shunt motor ; it may be started from low potential taps 
on the transformer connected to the alternating current 
side, the field circuit being open ; or it may be started by 
means of an induction motor mounted upon one end of 
the converter shaft. Each of these methods has its advan- 
tages and disadvantages, but as all of these methods are 
used extensively, it is obvious that the disadvantages are 
theoretical rather than concerned with operation. 

Starting from the Direct Current Side. — With this method 
the converter is started from the direct current side like a 
shunt motor, and the same precautions are taken as would 
be observed in starting a shunt motor of the same capacity 
as the converter. This method is used almost exclusively 
by fighting companies, as it requires a small amount of 



3IO EXPERIMENTAL ELECTRICITY 

apparatus and produces the least disturbance in the sys- 
tem. Furthermore, as a battery floats on many of the 
large systems, there is no necessity for the saving in time 
of starting that could be accomplished by other methods. 
To start, synchronize, and place full load upon a 1500-kw. 
machine, requires with a good operator about 90 seconds. 
Some companies prefer to start their converters from a 
separate motor generator set. When being started in this 
way, the converters do not accelerate so rapidly as when 
they are started from the direct current bus. They are 
then synchronized and connected to the alternating current 
side of the system. This method gives a flexible control, and 
the converter can run idly on the alternating current side as 
a synchronous motor until ready to be used, when it is con- 
nected to the direct current bus. Care must be taken in 
connecting the machine to the direct current bus to see 
that its voltage is slightly higher than the bus. If lower, 
there might be a tendency to motorize, the reverse current 
relays tripping the machine from the circuit. 

An interesting case was called to the attention of the 
writer in which the main power house was disabled, and it 
was necessary in order to receive power at a certain sub- 
station to send it from a small power station at a distance. 
The substation contained no storage battery, and the alter- 
nating current pressure when it arrived at the station was 
quite low. Not having direct current pressure on the bus, 
the operator connected the converter directly to the alter- 
nating current side, supplying low voltage to the alternating 
current, slip rings of the converter opening the field circuit. 
When the converter had reached synchronism, the field 
circuit was closed. This gave a direct current voltage 
which was lower than normal but which provided some 
low voltage current to the immediate territory. When the 



THE ROTARY CONVERTER 311 

trouble at the main power house was cleared the pressure 
on the alternating current mains was raised, and the direct 
current pressure became normal. 

Starting by Means of Induction Motor. — This method is 
used extensively by the Westinghouse Electric Manufac- 
turing Company. Upon the shaft of the converter is 
mounted the rotor of an induction motor, which is started 
from the main switchboard. The stator is mounted over 
the rotor suspended on a frame on the side of the converter. 
When the converter has been brought up to speed, it is syn- 
chronized. This method has the advantage that it requires 
a small amount of switch gear and less throwing of switches 
in starting than in the motor generator method, but its 
saving in time is offset by the lack of speed regulation nec- 
essary for rapid synchronizing where there is any voltage 
fluctuation. A good operator, however, with this method 
can synchronize and start a 1500-kw. converter in about 
two minutes. If much fluctuation of voltage exists, the syn- 
chronizing becomes more difficult. An experienced opera- 
tor endeavors to " catch" the machine on the way up. If 
the voltage on the system is below 15%, it may be impos- 
sible to start the converter, since the torque of an induc- 
tion motor varies as the square of the voltage. The 
principal advantage of this method is the increased factor 
of reliability, owing to the fact that each converter has its 
individual starting motor. If desirable, the converter can 
be arranged to start from the direct current side also, 
which can be used in case of emergency. 

Starting from the Alternating Current Side. — This is by 
far the simplest and quickest way of starting a converter. 
It is used extensively by railway companies where in 
emergency it is desirable to start a machine in the shortest 
possible time. The writer has started a looo-kw. machine, 



312 EXPERIMENTAL ELECTRICITY 

synchronized it, and had full load upon it, in 28 seconds. 
The operation was regularly accomplished by the opera- 
tors in the station in 33 seconds. 

The transformers supplying the alternating current side 
of the converter aie equipped with a series of low potential 
taps giving \ and f normal voltage. These various poten- 
tials may be supplied directly to the alternating current 
slip rings by means of two triple-pole, double-throw 
switches for a three-phase machine. At starting, the field 
circuit of the rotary is opened at about 6 points by a break- 
up switch, the double-pole switches are both thrown up, 
connecting the converter directly as an induction motor 
to the first set of low potential taps, and the rotary speed 
quickly rises to synchronism. When the operator thinks 
that synchronous speed has been reached, the field switch 
is closed, and the direct current armature voltage is noted. 
If the voltmeter tends to deflect in the wrong direction, the 
field switch is opened and closed in the opposite direction, 
in which case a pole will be slipped and the voltmeter will 
indicate properly. Sometimes the operator will continue 
closing and opening the field switch in one direction until 
the voltmeter indicates in the right direction. When the 
voltmeter indicates properly, showing synchronism, both 
double-throw starting switches are thrown down, raising 
the potential on the direct current side. This method owes 
its rapidity to the entire elimination of synchronizing, the 
converter operating as an induction motor up to synchro- 
nism, and operating as a synchronous motor when the field 
circuit is closed. It is necessary to use break-up field 
switches, as, in starting, high potentials are generated in 
the field coils, owing to the transformer action of the arma- 
ture turns, through which alternating current passes. At 
starting, a potential as high as 2000 volts across each indi- 



THE ROTARY CONVERTER 313 

vidual field coil is common. As has been said, the great 
advantage of this method of starting is its rapidity and 
simplicity, qualities highly desirable in time of emergency. 
Its disadvantage consists in the fact that at starting the 
armature takes from the system a heavy inductive load, 
which is likely to affect the regulation of the system. 
This load is frequently as high as the full load of the con- 
verter. An inductive load takes magnetism from the sys- 
tem, whereas the field coils of machines operating may be 
adjusted to supply magnetism to the system. The writer 
remembers a case in which a 1500-kw. converter, taking a 
normal starting current from the direct current side of \ 
full load value, was started from the alternating current 
side by being thrown directly upon the transformers. The 
starting current was 3 times full load value, or 12 times 
that which was necessary to start from the direct current 
side. These machines were constructed for good synchro- 
nous operation, in which case their self-induction was re- 
duced to a minimum. Converters intended to start from 
the alternating current side must have slightly higher self- 
induction in order to limit the starting current. These 
converters are further provided with series reactance coils, 
which, though installed for other purposes, — as, for in- 
stance, to allow sHghtly unbalanced potential to be placed 
in parallel, — serve to limit the starting current. When 
the self-induction of a converter armature is large, it acts 
in a sluggish manner on the circuit while operating, and 
does not permit the converter to respond quickly to sudden 
changes in frequency of the circuit, thus causing a tendency 
to Juint. 

The Hunting of a Converter. — The hunting of a con- 
verter is a term applied to the state of a converter arma- 
ture which has become slightly out of phase with the 



314 EXPERIMENTAL ELECTRICITY 

generator ; in attempting to get in phase it is carried 
beyond the neutral position, becoming out of phase again. 
This process of swinging back and forth across the neutral 
point is termed hunting. Where the hunting becomes 
excessive, the machine may be thrown out of step. When 
a converter armature is operating at synchronous speed, we 
can consider the armature to have definite poles north and 
south, being adjacent to the poles of the field winding. 
When hunting occurs, the poles of the armature tend to 
shoot across the field pole faces, the north poles of the 
armature tending to take the place of its south poles. 
In order to minimize the effect of hunting, pole dajnpers 
are placed on the poles of the converters. These pole 
dampers are copper grids, rectangular in shape, containing 
crossbars sunk into the pole faces. They act as the short- 
circuited secondary of a transformer to the converter arma- 
ture, neutralizing the self-induction of the armature. They 
also have eddy currents induced in them when the arma- 
ture flux tends to shoot across the pole face, and they 
therefore offer a resistance to the swinging of the arma- 
ture. They offer no resistance to the normal operation of 
the armature, except when it tends to go out of synchro- 
nism. 

Experiment 174. Open the field circuit of a small converter, i kw., 
and connect the alternating current slip rings of the machine to another 
converter. The second converter should be connected by means of a 
starting box to a direct current source of potential. Start this latter 
machine rather quickly, as the starting current will be excessive and the 
starting box may become quite hot. The i-kw. machine, being started 
from the alternating current side, will be operating as an induction motor. 
When this machine begins to ojDerate, the starting box handle of the first 
machine should be turned on the balance of its way rather quickly until 
all of the starting resistance is eliminated. The second machine will be 
operating as an induction motor and its field circuit may be closed, the 



THE ROTARY CONVERTER 315 

converter becoming a synchronous motor ready for its load. When a 
lamp load is placed upon the second machine, it will be operating as a 
converter. This method of starting is used in substation operation 
where a converter is being started for the first time, and it is desirable to 
dry out the circuits. The main generator is connected to a spare bus, and 
feeders connect it directly to the substation transformers passing through 
a spare station bus. The converter and generator are electrically con- 
nected together through the alternating current side of the converter, 
and the main generator is slowly started into operation, the converter 
slowly following in speed operation. The field circuit of the con- 
verter is opened. As the frequency of the generator is low, the e. m. f. 
generated is low. 

Synchronizing. — Synchronizing is the process of adjust- 
ing the frequency, the magnitude, and the phase displace- 
ment of two e. m. f.'s so that their wave shapes will coincide; 
accordingly, if the machines are generator and converter, 
the wave shapes of e. m. f. of the two machines will pass 
through their maximum and zero values at the same time 
intervals. The general practice when synchronizing is first 
to get the e. m.f.'s of the two machines adjusted so that 
they will have the same effective values. The speeds are 
next adjusted so that the frequencies of the two waves are 
the same ; and then by means of lamps, synchronoscopes, 
voltmeters, etc., the waves are brought together until 
there is no phase displacement. The two machines are 
then coupled in parallel. When syn- 
chronizing with lamps, they may be con- 
nected one in each phase, as in Fig. 399, 
a short-circuiting switch being arranged 
to be closed at the instant of synchronism. 
When the two e. m. f.'s are in synchronism, fig. 399. — Synchroniz- 
there will be no interchange of current i"g ^-ith Lamps. 
and the lamps will not light. In practice the lamps are 
connected in a secondary circuit, as Fig. 400, consisting 




3l6 EXPERIMENTAL ELECTRICITY 

of the secondaries of two transformers. The transformer 
secondaries may be connected in opposition, in which case 

Ul I the lamps will be dark at synchro- 
T nous speed, or the windings may 

tx^^J be connected to assist each other 

^j\N\ NWsN\ at synchronism, resulting in a 

bright lamp. Synchronizing with 



■■ a dark lamp is the more common 

Fig. 400. — Synchronizing with 

Lamp connected to Trans- method. Tungstcn lamps are 
formers. preferable to carbon lamps for 

synchronizing, as they remain luminous at a much 
lower voltage, a carbon lamp yielding no illumination at 
30 volts, whereas a tungsten lamip filament is just visible 
at about 10 volts. With the tungsten lamp the changes in 
luminosity due to change in e. m. f. are more rapid than 
with a carbon lamp, owing to the former lamp having a 
positive temperature coefficient. 

Synchronizing with a Synchronoscope. — In large central 
stations it is customary to operate the oil switch motor 
or the solenoid of the magnet type of switch from a 120- 
volt source of potential. This potential may be supplied 
from a small storage battery or from the regular service 
bus. The advantage of the separate source of potential 
is that in case of trouble the operator can readily manip- 
ulate his switches irrespective of load conditions. To 
operate an oil switch, open or closed, requires an inter- 
val of about -^ of a second. The operation of opening or 
closing the switch is performed by an operator who closes 
a small switch on the switchboard panel. It is evident, 
therefore, that if the operator is using the synchronoscope 
to synchronize, he must close the battery switch when the 
pointer of the synchronoscope is moving to the zero, and 
when for its particular rate of travel the pointer is away 



THE ROTARY CONVERTER 317 

from the zero such a distance that it will reach zero in -^^ 
of a second. It is better to connect the rotary to the circuit 
when it is going into phase instead of going slightly out of 
phase, for in one case the inertia of the armature assists 
and in the other case it retards. Too little attention is 
given to the question of inertia in operation. In starting 
a large machine into operation we must consider its inertia 
and not eliminate the starting resistance too rapidly. If 
the synchronoscope pointer is moving too rapidly over the 
dial, and the oil switch is closed at the instant of synchro- 
nism, the inertia of the armature will tend to throw the 
machine out of synchronism, the machine remaining in 
step, but sparking rather violently. 

Synchronizing with a Voltmeter. — A direct current volt- 
meter may be used to advantage to synchronize machines by 
connecting it in the circuit in place of a synchronoscope. 
The voltmeter needle will rise from zero and fall again re- 
peatedly as the machine is approaching synchronism, the 
motion of the pointer becoming slower and slower. When 
the needle is falling to zero very slowly, the operator closes 
the oil switch as the pointer has almost reached zero. This 
is a simple method and quite easy to manipulate. 

Synchronizing with Frequency Indicator. — With this 
method the voltage of both machines is adjusted until it is 
almost the same in each, and then the frequencies of both 
machines are made equal. The operator next places a 
lamp in the circuit and can usually get his machine in circuit 
on the first cycle of intensities on the lamp. 

Experiment 175. — Make a set-up between two small i-kw. converters, 
so that they can be started from the direct current side with resistance 
in each of the field circuits. Connect both of the three-phase slip rings 
of the machine through lamps and arrange a short-circuiting switch 
so that when closed each of the individual lamps will be short-circuited 
(Fig. 401). The purpose of the lamps, as has been said, is to indicate 



3i8 



EXPERIMENTAL ELECTRICITY 




such an interchange of current as is taking place. If two machines 

operating at the same syn- 
chronous speed were placed 
together with considerable 
phase displacement, there 
would be a heavy inter- 
FiG. 401. — Synchronizing Two Converters. change of current due to 

the difference in their in- 
stantaneous e. m. f.'s. This is repre- 
sented clearly by Fig. 402 where, if the 
switch were closed when the values of 
the two curves were at time interval 
A. 



one machine would be generating 




Fig. 402, — E. M.F. Relations when 
Machines are out of Phase. 



120 volts and the other machine would 
be zero potential, 120 having been se- 
lected for the maximum value of the 
curve. In continuing the experiment, 

start both machines and notice with short-circuiting switch open that 
the lamps rise and fall in candle power. When at a maximum of candle 
power, the machines have their greatest phase difference in instantane- 
ous potentials, being, in Fig. 402, 90° apart. If the switch were closed 
at this instant, the interchange of current would be at its maximum. 
Sometimes in synchronizing, the lamps will not go entirely out but will 
start to increase in candle power. A good operator never closes the 
short-circuiting switch the first time that the lamps pass through zero 
candle power; instead, he waits until two or three fluctuations have 
occurred in order that he may judge how much out of step his machines 
are. Then he closes the switch when he is sure that the light is going 
to pass through zero candle power and when it has almost reached zero. 
With the set-up described above study all of the features of synchronizing. 



Crossed Phases. — In order that two machines may be 
synchronized, it is necessary that all of the similar phases 
of the two machines should pass through their zero and 
maximum values at similar time intervals. Their respective 
rotating fields should each move in the same direction and 
at the same rate of speed. If two of the phases of one of 
the machines are crossed, as in Fig. 403, and an attempt is 



THE ROTARY CONVERTER 319 

made to synchronize, it will be impossible to get all of the 
phases of one machine to pass through their maximum 
values simultaneously with the phases of the other machine. 

The alternating e. m. f. of one machine 

will be tending to rotate the other pJ-j-o \ \\ 

machine in the contrary direction to *** ***' 

which the direct current service is fig. 403. — Crossed 
driving it. The two rotating fields in Phases. 

one machine will therefore be traveling in opposite 
directions. Try this experiment, connecting phase A to 
phase A\ phase B to phase C , and phase ^'to phase B\ as 
in Fig. 403. Start up both machines with the lamps in 
circuit, and attempt to synchronize. When the machines 
are traveling at speeds almost synchronous, it will be 
noticed that first one lamp will reach a maximum candle 
power, then a second lamp, and then a third. If the field 
rheostat is carefully adjusted, the lamps will light up first 
in 1-2-3 order and then, with further adjustment, in 1-3-2 
order. This experiment is both attractive and instructive. 
Make other combinations of the phases, keeping the origi- 
nal crossed connection, and notice that the machines can 
be placed in phase. 

Meaning of Unity Power Factor. — For a given output in 
a converter, there is a minimum current input which occurs 
when the converter is operating at unity 
power factor. Above or below this 
point, the input of the converter is 
greater than necessary for the given out- 
put, and the excess current unneces- 
FiG. 404. — Unity Power sarily hcats the armature. This fact is 
Factor of Converters, illustrated in Fig. 404. The adjustment 
to unity power factor is usually made by adjusting the 
field circuit which changes the self-induction of the system. 




320 



EXPERIMENTAL ELECTRICITY 



ww^ — ^AA/V^^, 




By adding or taking away magnetism, a resonant condi- 
tion is reached, in which the fixed capacity of the cables 
of the system is neutraUzed. At this point 
unity power factor is attained. This feature 
is of much importance, and all good operators 
endeavor to run their machines at unity power 
factor. 

a. c. volts X a. c. amperes x power factor = 
true watts. 

EI cos = W, where = angle of phase dis- 
placement between the e. m. f. and the current. 

Experiment 176. Connect up a converter so that it 
will operate from a 3-phase source of supply, converting 
Ex- it to direct current. Place an ammeter in one of the 
i^ phases and place a load upon the direct current side of 
the machine, Fig. 405. Note the reading of the direct 
current ammeter placed in series with the load. Vary 
the field excitation of the converter with the field rheostat, 
and notice that the direct current load remains constant but that the 
alternating current load varies, too much excitation or too little excita- 
tion causing an increase above the minimum value of the alternating 
current input. 

Converters operating in Parallel. — Two converters of 
the same size operating in parallel will tend to divide the 
load proportionally, provided that they are both set for 
unity power factor. There are several things which affect 
the distribution of load betw^een two such machines, namely, 
the temperature of the field coils, the adjustment of the 
field rheostats, and the condition of the brushes. When a 
converter is first placed in service on the bus after being 
idle for a day or two, the resistance of the field coils is that 
due to the temperature of the room. After the machine 
has been operating for a few hours the temperature of the 
windings increases, and this increases the field resistance 



Fig. 405 

periment il- 
lustrating 
Unity Power 
Factor. 



THE ROTARY CONVERTER 32 1 

and decreases the field excitation, making necessary a 
readjustment of the field rheostat. This change in resist- 
ance may be noted by the operator from the fact that the 
other converter will gradually take most of the load. Ad- 
justment of the field rheostats will distribute the load at will 
between the machines in a station and between separate sta- 
tions. This adjustment alters the power factor, and varies 
shghtly the e. m. f. of the converter. The writer, when 
operating at night two 1500-kw. converters, has observed 
the following interesting fact. When the load would fall to 
such a point that it was felt that one machine was sufficient, 
one of the converters was disconnected from the bus. 
Upon affecting this disconnection it was always noticed 
that the previous load was not now carried by the single 
machine, but that part of the load seemed to be distributed 
to other stations, although no adjustment of the field 
rheostat of the machine remaining in service had occurred. 
The remaining load was always about two-thirds of the 
original load. It was possible, however, by a readjustment 
of the field rheostat, to bring the original load back to the 
station, but in this event it was necessary to give the con- 
verter a leading power factor. It is very important to en- 
force a general rule requiring that all machines should be 
operated on a system at unity power factor, for in this case 
the whole system operates at its highest efficiency. The 
condition of the brushes has much to do with the manner 
in which a machine will take its share of the load and will 
sustain overloads. A machine whose commutator has been 
carefully sand-stoned, whose brushes are clean, and whose 
brush tension is not over 3 lb., will oftentimes easily sus- 
tain overloads of 100% for considerable periods of time, 
whereas if the machine is not in good condition it will spark 
badly with slight overloads. 



322 



EXPERIMENTAL ELECTRICITY 



Recent Developments in Converters. — Several new devel- 
opments in converters have been made during the past 
few years. These consist of the vertical rotary, the rotary 
with compensating poles, and the rotary with interpoles. 
The vertical rotary is similar in construction to the ordinary 




Fig. 406. — Booster Converter (Westinghouse). 

rotaries, except that its shaft is vertical instead of horizon- 
tal, supported upon a special form of roller bearing. The 
main advantage claimed for this machine is that it has less 
weight, that it occupies less space, and that the brushes 
can be got at more readily than with the ordinary machine. 
The compensating pole converter, or the booster converter, 
Figs. 406, 407, 408, was developed with a view to ehminating 
the induction regulator from electric lighting circuits. The 
induction regulator is used to raise or lower the voltage on 
the alternating current side of the machine. The regula- 
tor has two windings, a series and a shunt winding — the 
phase of a fixed e. m. f. in the series winding is changed, 



THE ROTARY CONVERTER 



323 




Fig. 407. — Booster Converter (Westinghouse). 




FiG. 408. — Booster Converter. 



324 



EXPERIMENTAL ELECTRICITY 



adding the e. m. f., vectorially increasing or decreasing the 
resulting line voltage. The converter of the booster type 
has an additional series of poles surrounding the armature, 

these poles being 
outside of the regu- 
lar poles, as shown 
in Figs. 407, 408. 




COMPENSATING 
WINDING 



Fig. 409. — Position of Zero Boost on Converter. 




COMPENSATING 
WINDING 



Fig. 410. — Boost in a Positive Direction on Converter. 



These auxiliary 
poles are excited 
from a pair of con- 
tacts, which move 
over a resistance 
shunted across the 
potential circuit. 
When the contacts 
are opposite each 
other, as in Fig. 
409, no difference 
of potential exists between A and B, and no current passes 
through the auxiliary winding, thus producing a zero boost- 
ing effect. When the con- 
tacts are as in Fig. 410, 
the current passes through 
the field winding in one 
direction, and when they 
are as in Fig. 411, the cur- 
rent passes through the 
field winding in the oppo- 
site direction, boosting in FIG.411. — Boost in a Negative Direction 
, 1.1 or Crushing Effect in Converter. 

one case, and crushmg the 

voltage in the other case. As the same armature winding 
is used for both the regular field winding and the booster 
field winding, the booster e. m. f. will be alternating in char- 



V 


8 

— 




. 




A 




8 


Uy^Jv.^ 1 


B ^ 


^ 



THE ROTARY CONVERTER 



325 



acter, and will have its zero and maximum values occur at 
the same time intervals as the regular armature e. m. f., or 
will be in phase with it, simply varying its effective value as 
the booster field winding is varied. They have been a great 
improvement over the converters equipped with induction 
regulators, as the latter occupied considerable space, were 




Fig. 412. — Interpole Converter. 



difficult to repair, and did not give so large a voltage varia- 
tion as the booster converter. It is important to remem- 
ber, in installing the booster converter, that the variable 
potential resistance for the booster fields should have suffi- 
cient contacts so that the potential of the converter will 
not be varied by too great steps. 

The interpole converters consist of a series of small 
poles called interpoles, which are placed alongside of the 
larger poles, as in Figs. 412, 413, and which result in increas- 
ing or decreasing the polar span. These machines were 
developed by the General Electric Company. They per- 
form the same function as the auxiliary poles of the booster 



326 



EXPERIMENTAL ELECTRICITY 



converter, that is, they raise or lower the direct current 
voltage ; the result, however, is accomplished in a slightly 
different manner. In one case the main field flux is 
varied, while in the other case two e. m. f.'s are superim- 
posed upon each other. 





IlKrl 


-^ m^w , ^: 




BH^^HMHi 



Fig. 413, — Interpole Converter. 

Rotary Converters versus Motor Generators. — Without 
doubt the use of the motor generator does much to im- 
prove the power factor on systems carrying a large induc- 
tion motor load. The equipment, however, for the same 
conversion of kilowatt from alternating to direct is greater 
for the motor generator than for the converter equipment, 
since a synchronous motor or an induction motor with suit- 
able switch gear is necessary. With the synchronous motor 
the operator has good control of the power factor, but 
loses time in synchronizing. With the induction motor, 
no control of power factor is possible, but the machine will 
not fall out of step so easily as it does with a synchronous 
motor. It may be well to mention, also, that with the 
converter a minimum space is required, especially where 



THE ROTARY CONVERTER 327 

the induction regulator is not used, that the range of volt- 
age variation is not so great as with the motor generator, 
and that the general improvement in power factor on the 
system is nowhere near so large. Converters should be 
used for city work where the direct current load is heavy, 
and where little alternating current is used for an induc- 
tion motor load. For large outlying territory, where much 
alternating current inductive load is used, and where the 
voltage conditions are variable, owing to the absorption of 
many small plants, the motor generator has been found to 
give excellent service. This system has been used by the 
Boston Edison Company with great success. If it were 
possible to obtain rotary converters which would operate 
satisfactorily on 60 cycles, it is possible that they would be 
used in many places where the motor generator is now 
installed. 

QUESTIONS 

1. How is a rotary converter constructed ? 

2. What is meant by the term synchronizing? Give several 
methods. 

3. Why is it desirable to operate a converter at unity power factor? 

4. Give the relative advantages and disadvantages of the different 
methods of synchronizing. 

5. When starting from the alternating current side with field cir- 
cuit open, why can the operation of a converter be considered as equiva- 
lent to that of an induction motor? 

6. Why does a change in the field resistance of a converter from 
temperature cause a change in the power factor ? 

7. What is meant by the hunting of a converter, and how may 
the etfect be minimized? 

8. Given one converter having four poles, and the other having 
eight poles, what will be their relative speeds when in synchronism 
with the same source? 

9. Why is the relation between alternating e. m.f, and direct cur- 
rent e. m. f. fixed for a converter? 

10. Derive the relation of direct and alternating e. m. f.'s for a three- 
phase machine. 



APPENDIX 

EXPERIMENTAL PROJECTION APPARATUS 

(For the Teacher) 

Where many students have to be instructed simultaneously, where 
the time is limited in which a given subject must be taught, and where 
the ground to be covered is extensive, there is no better method of 
instruction than with experimental lectures. While it is true that it 
requires about five hours to arrange the apparatus for one hour's lecture, 
the benefits which the student derives from such lectures far outweighs 
the extra labor in their preparation. Psychologists have shown that 
with the experimental method of presentation, the ability of the student 
to remember is increased about tenfold. This result is due to the fact 
that the student's ability to observe is increased, and that new forms of 
memory association arise from seeing the experiments performed. The 
student not only gets a better conception of various physical phenomena, 
but he understands and is able to remember better the fundamental 
principles of the science. 

It is interesting in developing the experimental method of presenting 
the subject of electricity, to note to what extent this method may be 
employed. So far, the author has been unable to find any experiment 
ordinarily performed in the electrical laboratories which cannot be per- 
formed on the lecture table, in such a manner that all readings of 
instruments can be made by the students, and all calculations can be 
readily followed. An instance of this may be cited, namely, the measure- 
ment of the power factor of an alternating current arc lamp. The arc 
lamp is operated from an alternating current source of supply, the volts, 
amperes, and watts of the lamp circuit being given by means of a project- 
ing alternating current ammeter, a projecting alternating current volt- 
meter, and a projecting wattmeter. This requires two projecting lan- 
terns, as the voltmeter reading, which is constant, is taken first, then 
an ammeter is substituted for it, the wattmeter reading being projected 
by means of the other lantern. For ordinary work an experimental 
lantern having large condensers, 6", with a combination vertical and 

329 



330 



APPENDIX 



horizontal attachment, provided with detachable separate condensers, 
having binding posts for electrical pressure, also a small table to hold 
electrolysis tanks, prisms, and instruments, is especially desirable. In 
case there may be some who are not acquainted with the details of an 
experimental lantern, one designed by the author, and built by Beseler 
& Co., of 251 Centre Street, New York, is described. 

The Experimental Lantern. — The experimental projecting lantern, 
Figs. 414, 41 5, used by the author, is somewhat similar to the ordinary col- 
lege lantern, Fig. 416, except that the lantern is equipped with extra large 




FkJ. 414. — Experimental Lantern arranged for Horizontal Projection. 

detachable condensers, which may be quickly removed during a lecture. 
In building this lantern a simple patented device was used which held 
the condensers in position so that either condenser could be removed 
independently of the other, one condenser being removed from the front,^ 
the other from the side. With this arrangement the condensers have so 
much freedom in their supports that they never break, although a 25- 
ampere arc is often used in the lantern. The lantern box is built of 
sheet iron, with special arrangements for ventilation, to which the con- 
denser holder is rigidly attached. The condensing lenses holder of 



APPENDIX 



331 




Fig. 415. — Experimental Lantern arranged for Vertical Projection. 




Fig. 416, — Single Lantern. 



332 APPENDIX 

such a lantern should not be mounted on a separate support, as it 
interferes with the use of the vertical attachment. 

An automatic lamp, Fig. 419, is used with this lantern which can be 
turned on and off, either at the lantern or at the lecture table. This 
lamp, Fig. 419, operates at about 18 amperes, this being sufficient cur- 
rent to use. The lamp is mounted upon a sliding support so that it can 
be moved forward or backward about an inch, and also raised. Al- 
though a right-angled feed lamp. Fig. 417, gives a better projection than 
the ordinary automatic vertical feed lamp, there are no automatic right- 
angled feed lamps on the market that are satisfactory for this work. 
When properly adjusted the vertical lamp gives an excellent projection, 
the light remaining steady while in use. 

In front of the lamp house is mounted a rectangular base. Fig. 414, 
which supports the various objects placed before the lamp house. These 
consist of a lens holder which carries a \ size Darlot lens, so that the 
lantern can be operated about 15 feet from the lecture table. This is nec- 
essary, as one must continually pass to and fro from lantern to lecture 
table during a lecture. Some lecture rooms are arranged so that the 
lantern can be placed on the lecture table and may project on the side 
wall. This arrangement is somewhat more convenient for a large room. 
Another support consists of an adjustable table about 3'' in width and 
6" long, which can be raised or lowered. Projecting instruments, tanks 
containing liquids, as for electrolysis experiments, small electro-magnets, 
can be placed upon the table close to the condensing lens, and projected 
on the screen. On the lantern box over the condensers are two double 
binding posts mounted upon insulated supports. These can be used in 
supplying current to small devices which have to be operated before the 
lantern. The slide carrier has four tapering pins upon it which fit into 
four holes on the condenser. The vertical attachment is supported by 
a table in front of the condensers, the outer condenser being removed 
and placed in the top of the vertical attachment. The objective lens is 
mounted in a support on the vertical attachment. A small pivoted 
mirror directs the image on the screen to any desired point. The ob- 
jective lens is clamped in position by a screw. It is possible to change 
over the horizontal lantern to a vertical lantern in one minute during a 
lecture. A 45° mirror is mounted inside of the vertical attachment, 
where it is protected. 

How to operate an Electric Projecting Lantern. — An outfit for an 
electric projecting lantern comprises the following apparatus : one arc 



APPENDIX 



333 



lamp, either hand or automatic feed ; one rheostat, either adjustable or 
fixed ; one lantern box with appurtenances ; one double-pole switch 
with fuse ; one coil of flexible wire; and two carbons. 

This apparatus should be connected up with insulated wires in the 
manner indicated in Fig. 417, so that the arc lamp can receive electric 



Poutive 




Me^hvs 




^i 



Ith^osfcff. 



j^r. 



source. 



Fig. 417. — Lantern Set-up. 



current from a suitable source. When selecting a proper service upon 
which to operate an electric lantern, care should be taken to see that 
the service wires are of sufficient capacity to carry the lantern current 
without danger of becoming overheated and causing fire. An operator 
should never seek to obtain lantern current by means of an attachment 
plug connected in a lamp receptacle, a cluster, or a lamp circuit of any 
kind ; the electrical connection must be made at a regular outlet or a 



334 APPENDIX 

junction box connected in on the mains. Data on this point may be 
found in the Fire Underwriters' rules. 

When the set-up is being made for direct current, care should be 
taken to note that the positive lead is connected to the upper carbon. 
Whether the connections are correct or not can readily be determined 
by allowing the lamp to burn for a short time, then opening the circuit, 
and observing which carbon is the brighter as they cool, for the posi- 
tive carbon will remain bright longer. If the positive carbon should 
happen to be the lower one, the connecting wires can easily be reversed 
at the lamp terminals. 

To operate the lamp ordinarily, see that there is electric pressure be- 
yond the fuses. If it be a low voltage circuit of ii6 volts this can be 
readily determined either by connecting an incandescent lamp across 
the mains or by moistening the thumb and forefinger of one hand and 
placing them across the circuit. A tingling sensation in the fingers will 
indicate the presence of volts. The latter method is not advisable un- 
less the voltage of the service is known, for if an operator were to try 
this on a 500-volt service it would probably burn his fingers severely. 
When there is pressure beyond the fuses, separate the carbons. Then, 
if a hand feed lamp is used, set the handle of the adjustable rheostat in 
such a position that all of its resistance is in the circuit, and close the 
main switch. The carbons may then be brought into contact by turn- 
ing the handle of the lamp mechanism. When the carbons are in con- 
tact they should be slowly separated, turning the lamp handle in the 
opposite direction. The rheostat handle should then be moved to such 
a position that the desired current passes through the arc. As the 
carbons burn away they should be fed together gradually, the distance 
between the arc for a right-angled hand feed lamp being about \ of an 
inch. If the upper carbon extends over the lower one too far, the upper 
carbon will not burn away properly but will form a long tip, causing the 
arc to sputter. 

With automatic lamps manipulation of the carbons is unnecessary. 

Theory. — Intelligent operation of an electric lantern requires the 
knowledge of the application of Ohm's law. 

It is customary when describing the general application of this law 
to use the term " an electric circuit." An electric circuit consists of a 
complete conducting path from one terminal of an electric generator, 
through the consuming device, to the other terminal of the generator. 
By means of a system of copper feed wires, the terminals of the main 



APPENDIX 



335 




Fig. 418. — Two Lamps in Series, 

How to connect Two Lamps in Series. 

(i) If it is desirable to operate a dissolving lantern from a ii6-volt service 
which is wired with a carrying capacity of about 18 amperes (wire No. 10), two 
lamps may be connected in series with one rheostat adjusted to pass a current 
not greater than one lamp. 

(2) Two lamps may be operated in series with two rheostats from a 220-volt 
service, as Fig. 418. 

Never use wire for arc lamps smaller than No. 10 B. & S. 



2^2,6 APPENDIX 

generator at the power house are connected directly to outlets in many 
buildings in large cities. In some cases each building is equipped with 
its own generators located in the sub-cellar, to which the outlets are 
wired directly. These outlets are usually protected with a fuse and a 
switch, the function of the switch being to disconnect the consuming 
device connected to it, and the function of the fuse being to limit the 
energy supply. If the demands of the consuming device be greater 
than the capacity of the fuse, the fuse will melt, opening the circuit. 

The application of Ohm's law to a lantern circuit may be shown as 
follows : Consider the electrical connections for a single projecting lan- 
tern. Fig. 417. They consist of the mains (source) coming from the 
main generator, terminating in the fused double pole switch. From 
one terminal of the switch a wire connects to one of the carbons of the 
lamp, the other terminal of the switch being connected to one of the 
terminals of the rheostat. With the switch open, the remaining termi- 
nals of the rheostat and the lamp are connected together. The switch 
is then closed and the carbons are adjusted until they touch and form a 
spark. The carbons are then separated about an eighth of an inch, 
producing a brilliant arc. Assume that the resistance of the rheostat 
is such that a current of 15 amperes is passing through the circuit. 
Assume also that the pressure of the circuit at the source is 120 volts. 
Under ordinary operating conditions an arc requires a pressure across 
its terminals of 40 volts for proper operation. If the arc consumes 40 
volts and the resistance across the whole circuit is 120 volts, obviously 
the resistance must consume 80 volts. 

If a current of 1 5 amperes passes through a resistance and the pres- 
sure across its terminals is 80 volts, what, according to Ohm's law, will 
the resistance be ? The answer is 5.33 ohms, obtained by dividing 80 
by 15. If we were to measure the voltage across the carbons of the arc 
with a voltmeter, it would here be 40 volts, while the voltage across the 
resistance would be 80 volts. It must be remembered that the arc 
usually consumes 40 volts, and this must be subtracted from the total 
voltage to obtain the net voltage consumed by additional resistance. 
Thus, on a 220-volt circuit, 180 volts must be consumed; on a 50-volt 
circuit 10 volts must be consumed, and on a 500-volt circuit, 460 volts 
must be consumed. 

Having determined the voltage which must be consumed in any 
case, the proper resistance to limit the flow of a given current can be 
obtained directly from Ohm's law. Thus, given an arc lamp fed 



APPENDIX 



337 



through a resistance from a 120-volt circuit, what resistance is neces- 
sary to pass currents of 15, 18, 20, and 25 amperes? We proceed 

thus : 

Total pressure, . . 120 volts 

Lamp pressure, . . 40 volts 

Remaining pressure, 80 volts 

For 15 amperes, 80/15 = 5.33 ohms 
For 18 amperes, 80/18 = 444 ohms 
For 20 amperes, 80/20 = 4.00 ohms 
For 25 amperes, 80/25 = 3.20 ohms 

In a similar manner other conditions may be worked out. 

Hand-feed Arc Lamp. — A hand-feed lamp is a mechanism consisting 
of two movable arms which are insulated from each other. These two 
arms have holes in their extremities which contain carbons. These 
carbons are so supported 
by the arms of the mech- 
anism as to be able to move 
in the same plane. The 
support to the arms is usu- 
ally in the form of a screw, 
so that the carbons may be 
fed together at a definite 
rate as they are consumed. 
When burning upon direct 
current, the positive car- 
bon, which is the upper 
one, burns away twice as 
rapidly as the negative car- 
bon. It is therefore ob- 
vious that the mechanism 
must feed the positive car- 
bon twice as rapidly as the 
negative carbon. When 
using alternating current, 
however, both carbons are 
consumed at the same rate, 
and consequently both carbons must be fed together at the same rate. 
The improved Sunray hand-feed lamp. Fig 417, is arranged with inter- 




FlG. 419. — Automatic Arc Lamps. 



338 



APPENDIX 



changeable gears so that it can be used either on direct or alternating 
current. For instance, as the gears are set on cut, the lamp is arranged 
for direct current. If, however, it is desirable to use alternating current, 




Fig. 420. — Combination Beseler Double Lantern and Moving Picture Apparatus. 

one thumbscrew is released, another thumbscrew is unscrewed, and its 
screw is removed. It is then possible to move the upper carbon holder 
forward so that its gear meshes with the inner gear on the lower carbon 
holder. This gear has the same number of teeth as the upper gear, and 
consequently both carbons may be fed together at the same rate. 



APPENDIX 339 

Rheostats. — The function of a rheostat is to limit the supply of 
electrical energy furnished an arc lamp, and also to help maintain a 
steady arc. It has been found undesirable in practice to operate an 
arc lamp directly upon a service without resistance in its circuit, since 
the resistance of the arc varies over such wide ranges of values. When 
the carbons are in contact the resistance of the arc is practically zero, 
except for what may be due to the structure of the carbons. If in this 
condition the arc lamp were connected up without resistance to a low 
voltage circuit, it would pass an excessive current. 

A rheostat for lantern use is usually wound of resistance wire coiled 
up in the form of a helix. This wire must have sufficient carrying 
capacity so that it will not heat excessively. Its temperature should 
not rise above 550° F., and the whole structure of the rheostat must 
be mounted in a sheet-iron enclosing frame. The resistance wires of 
the rheostat should be insulated from the framework of the rheostat 
at the points at which they are supported, so that the framework will 
not form what is termed a short circuit. 

The most practical rheostats for projection work are made variable, 
to increase or decrease the amperage according to conditions that may 
arise. 

Moving-film Apparatus. — It is sometimes convenient in using a 
double lantern, to have it arranged with a moving-picture attachment, 
especially when showing films of e. ra. f. and current phenomena. A 
convenient apparatus made by Beseler & Co. for this purpose is shown 
in Fig. 420. As the moving-picture attachment requires a more 
powerful light, the lower lamp house is arranged to slide back and forth 
to be used in operating the film apparatus or as the lower half of the 
double lantern opposite. 

Projecting Electrical Instruments. — Throughout the text reference has 
frequently been made to four electrical projecting instruments, a galva- 
nometer, a wattmeter, an alternating current voltmeter, and an alternating 
current ammeter. All of these instruments. Figs. 421, 422, were built by 
Dr. Edward Weston for projection. The alternating current ammeter is a 
self-contained lo-ampere instrument, the alternating current voltmeter is 
a 150-volt instrument, and the wattmeter is a regular 300-watt labora- 
tory instrument arranged with a transparent scale. The galvanometer is 
only 5 inches in diameter, and has a transparent scale graduated in 50 
parts, central zero, with zero shifting device. This instrument is made from 
a specially designed Weston movement, possessing all of the character- 



340 



APPENDIX 



istics of this type of permanent magnet instrument. The galvanometer 
may be used to show induction experiments, or it may be used with 
shunts as an ammeter or with resistance as a voltmeter. A double- 




FlG. 421. — Weston Projecting Instruments. 

throw double-pole switch is usually arranged so that the galvanometer 
terminals come to the middle of the switch ; the switch when thrown 
in one direction indicates volts, and when thrown in the other direction 
indicates amperes. For speed measurements a Weston speed tachome- 




FlG. 422 — Combination Board used with Projecting Galvanometer. 

ter is belted to the apparatus under test, and the magneto terminals 
are wired to the galvanometer with resistance in the circuit. This 
resistance is varied until a uniform relation is obtained between speed 



APPENDIX 341 

and deflection. This arrangement is particularly advantageous when 
the lecturer is discussing the operating characteristics of motors. For 
temperature work the galvanometer may be connected to a thermo 
element. Great accuracy can be obtained by having two junctions, such 
as copper to nickel to copper, and placing one junction in water, and 
the other junction in contact with the temperature point that is being 
measured. The difference in temperature between the water and the 
temperature point is indicated by the galvanometer. The thermo 
e. m. f.'s corresponding to various combinations of metals may be found 
in Thomson's Elementary Lessons in Electricity. 

As a suitable adjunct to this text the author recommends Sloan's 
Elementary Electrical Calculations, in which will be found numerous 
practical problems. 



FORMULA 

Page 

i=K^^e 30 

^^^Y.N^S \ ^g 

Circular mil = diameter in mils squared. 

One mil = jJ^o of an inch 61 

Resistance = '-^^. r^^-j r;-- 62 

cross section in circular mils 

R = r\-\-ri. Series resistances - . . 62 

R = ^—^ — . Multiple resistances 63 

7?^=i?Q(i -f .0042/). Temperature coefScient 63 

E = 1.4328 — o.ooii9(/— 15° C.) — 0.000007 (/— i5°C.)2. 

E. m. f. standard Clark cell 68 

E= 1.01985 volts. E. m. f. Weston standard cell. . . 68 

H- - 

^ = f- 73 

E=IR n 

e:e'::R:R'. Distribution of potential 74 

M=Izt. Faraday's electrolysis formulae 103 

E 
R = -±-. 125 

F 

R' 

e'.e'::R:R + R'. Calibration of voltmeter 136 

A:B\:0:x. Slide wire bridge 141 

X=C^<\>{S' -S). Carey-Foster Bridge 144 

X= C — <f) {S' — S). Carey-Foster Bridge 144 

A\B\\C:D\\E:X. Thomson Double Bridge. . . .144 

K= ^- Constant of galvanometer 148 



r X " * 



342 



FORMULA 343 

Page 
C: C '.:6:9'. Measurement of Capacity. . ...... 149 

I— Motor Armature Current icq 

Actual revolutions x ico ^ . r ^ 

— : = percentage of accuracy of meter. 229 

allotted revolutions 

A + B + 2>C 

— =z average per cent accuracy of wattmeter. . 235 

„^ , , max. max. 

Effective value = — =- = 2Co 

V2 141 

U = I'^ Rt y. .2^. Heat developed in a circuit 251 

Maximum = - X average = 1.57 X average 252 

„ „ effective 

F. F. = 

average 



^E 



max. 



\/2 

F. F. = =1.11 252 



max. 



I=2'jrfCe, 1= Capacity alternating circuit. . . 257 

2 7r/C 

c = Condensers in series 21:0 

1 + 1 + 1 '^ 
^1 c'^ c^ 

Es——L{—y E. m. f. of self-induction 260 

r _ flux X number of turns _ (j>N' . 
current X 10^ IV2X 10^ 

^^'^-•=^' ^^^ 

Ef=2'nfLl= effective e. m. f. of self-induction. . . . 263 

sin 30° = 1 = .5 = cos 60° 269 

cos 30° = ^- = .866 = sin 60° 269 

tan 30° = -i-. 269 

^3 



344 FORMULA 

■yJ~L Page 

tan 60^ = ^- 269 

. ^ iTrfLI itrfL 

sin B — = ^ • .... 270 

/ ^R^ + (2 tt/L) 2 Vy?2 + (2 TzfL) 2 

cos6l = - — ^^ = — ^ 270 

/ ^R' + 2lTf VR-2 + (2 7r/Z)2 

^ ^ 2 7r/Z 2 7r/Z .L 

z=^//e^+(../z__L_)^ .7. 

Resonance when 2 tt/Z = 272 

2 7r/C 

/ = — = ^ . . 272 

^5 = AD X cos ^, cos 6 = power factor 275 

F=Ex/x cos 276 

. = power factor 276 

Ex/ ^ 

tan = ^2 ^ ~ • Tangent formulae three-phase power 

PFi + W-2 

circuit 277 

/^i^ cos (^ + 30°) 277 

W-i cos(6»-3o°) 



INDEX 



Abscissa, 12. 

Acid solutions, 103. 

Admittance, 266, 272. 

Aging of magnets, 18. 

Air gap, 6. 

Alkalies, iii. 

Alternating currents, 49. 

arc, 186. 

circuit, 195, 270. 

generators, 252. 

principles of, 242. 
Aluminium, 113. 
Ammeters, 17, 19. 

calibration of, 129. 

calibration of , series method, 131. 

shunt, 22. 

Thomson inclined coil, 41. 

voltmeter method of measuring re- 
sistances, 125. 
Ampere, 59. 
Arc lamps, 186. 

circuits, 192. 

hand-feed, 337. 
Armature, circuits, 155, 159. 

cross section, 40. 

field of series motor, 175. 

of motor, 37. 

shunt connection, 170. 
Attraction and repulsion of magnets, 2. 

Balance coil, for arc lamps, 193. 

set for a. c, e. m. f.'s, 246. 
Balances, 51. 

Ballast resistance, life of, 220. 
Bariimi cyanide, 116. 

.hydrate, 116. 
Bleach, 11 1. 

Bliss Car Lighting System, 53. 
Blow-out magnets, S3- 
Booster converter, 322. 
Box negative, 97. 
Branch circuits, 124. 
Brass plating, no. 



Bridge control, 182. 
Bunsen cell, 86. 
Busses, 26. 
Buzzer, 35. 

Cables, 26. 
Capacity, 242, 254. 

current relations, 257. 

effect, 255. 

formulae, 257. 

measurement of, 149. 

reactance, 257, 272. 
Carbon arc, 188. 

incandescent lamps, 202. 

physics of, 188. 
Carbonizing filament, 206. 
Carbons, 186. 
Carborundum, 115. 
Carey-Foster bridge, 143. 
Cell, Daniell, 83. 

closed circuit, 83. 

e. m. f.'s, 79. 

open circuit, 87. 

simple, 77. 
Characteristic curves, shunt motor, 178. 
Chemical action of cell, 78. 
Chloride cells, 96. 
Circuits of shimt motor, 153. 
Circular mil, 61. 

Coefficient of self-induction, 260. 
Coherer, 55. 

Color of illuminants, 197. 
Commutator of motor, 39. 
Compass, 2. 

Compensating coil, T. R. W., 222. 
Condenser, in, 258. 

electrolytic, 82. 
Conductivity of PbOo, PbS04, 91. 

copper sulphate, 102. 

sodium acetate, 141. 
Contact maker, 244. 
Controller, testing, 184. 

series multiple, 180. 



345 



346 



INDEX 



Converter, booster, 322. 

in parallel, 320. 

interpole, 322. 
Cooper-Hewitt tube, 216. 
Coordinate paper, 122. 
Copper electrode, 85. 

plating, 108. 

wire table, 65. 
Cosine, 268. 

Counter e. m. f. of motor, 159. 
Critical current density, no. 
Current, 59, 69, 70, 71. 
Cycle, 243. 

of magnetism, 10. 

Daniell cell, 83. 

Davy's discovery of arc, 186. 

Daylight lamp, 196. 

Depolarizers, 81. 

Dip of magnet, 4. 

Direct method of meter test, 228. 

Double current generators, 49. 

Dry cells, 88. 

Drying filaments, 205. 

Dynamotor, 50. 

Eddy currents, 18, 53, 223, 
Edison bottle meter, 221. 

cell, 95. 

lamp, 203. 

three- wire system, 119. 
Effective values, 250. 
Efl&ciency of illuminants, 191. 
Electric bell, 34. 

furnace, 114, 
Electro-chemical equivalents, 103. 

dynamometer, 29. 
Electrodes, effect of changing, 79. 
Electrolysis, loi. 

of water, 105. 
Electrolytes, defined, loi. 

effect of changing, 80. 
Electrolytic condenser, 82 . 

interrupter, 117. 

products, III. 

rectifier, 116. 
Electro-magnet, 32, 16. 
Electro-motive force, 59. 

generation, 47. 
Electroplating, 108. 
Emergency brake, 183. 



Enclosed arc, 195. 
Exide battery, 98. 
Experimental coil, 27. 

projection, 330. 

tank, 78. 

Faraday, Michael, 46. 
Feeder ammeter, 131. 
Field coils of motor, 37, 154. 

rheostat, 167. 
Filament, characteristics of, 209. 

mount, 208. 

treating of, 206. 
Flaming arc, 198. 
Flashing filament, 204. 
Flux, 6. 

density, 6. 
Form factor, 252. 
Foucault currents, 53. 
^riction, of recording wattmeter, 227. 

curves of meter, 222. 

Galvani's experiment, 77. 
Galvanometer, 145. 

determination of constant, 149. 

how to set up, 147. 

resistance of, 147. 

shunt, 140. 
Gears, T.R.W., 223. 
Gem lamp, 209. 
Generating e. m. f.'s, 244. 
Generator, 48. 
Gillette's safety razor, 15. 
Glower for Nernst lamp, 219. 
Gold plating, 109. 
Graphite, 114. 
Gravity cell, 85. 
Grenet cell, 86. 
Ground, resistance of , 171. 

tests, 169. 
Grove cell, 86. 

Helix, 27. 

Holophane reflector, 198. 
Horseshoe magnet, 4. 
Hunting, 313. 
Hydrometer, 94. 
Hysteresis cycle, 9. 
loss, 10. 

Inductance, 242. 



INDEX 



347 



Inductance formulae, 261. 
Induction, 28. 

coil, 55. 

generator, 253. 

magnetic, 7. 

motor, 38, 299. 

motor, care of, 305. 

motor, operation, 302. 

motor, rotation, 305. 

poles, 301. 

wattmeter, 279. 
Inductive e. m. f. and current relation, 
265. 

reactance, 272. 
Infra rays, 190. 
Inspection tests of motor, 234. 
Installation tests, 233 . 
Insulation test, 144. 
Interpole converters, 322. 

motors, 172. 
Intrinsic brightness, 189. 
Iron core in helix, 31 . 

Keeper for magnet, 9. 

Laminated shields, 23 . 
Lamp board, 130. 
Leakage, magnetic, 6. 
Leclanche cell, 87. 
Limit switch, 29. 
Lines of force, 6, 46. 

effect on compass, 26. 
Load, 236. 

box, 232. 
Local action, 84. 
Lodestone, i. 
Low voltage obtained, 75. 

Magnet, i. 

horseshoe, 4. 

of Weston voltmeter, 5. 

pole, 2, 15. 
Magnetic circiiits of motor, 156. 

of series motor, 175, 

dip, 4- 

field, 14. 

around wire, 25. 

pole, 3. 

leakage, 6. 

induction, 7. 

spectrmn, 2. 



Magnetism, 1-24. 

molecular theory, 6. 
Magnetite, i. 

arc, 199. 
Magnetization curve, 23, 51. 
Magnetizing force, 14. 
Manchester positive, 98, 
Mariner's compass, i. 
Maximum a. c. value, 250. 
Mayer's needles, 16. 
Measurement of power factor, 276. 
Mechanical equivalent of light, 189. 
Metallic salt solution, 107. 
Meter installation, 238. 

service, 237. 

testing, 227. 

wiring, 241. 
Molecular magnets, 16. 

theory, 6. 
Moore vacuum tube, 214. 
Motor, 37. 

generator, 49, 50, 326. 
Moving-picture apparatus, 338. 
Mutual induction, 53. 

Negative temperature coefficient, 209. 
Nernst lamp, 218. 
Neutral plane, 157. 
Normal load, 235. 
North magnetic pole, 3. 

Ohm, 59, 60. 

law, 71, 72, 73, 74, 126. 
Ordinate, 12. 
Oscillograph, 248. 
Overload relays, 29, 

release, 163. 

Pail forge, 118. 
Parallax, 22. 

Partial hysteresis curve, 13 . 
Permeability, 10, 31. 
Phosphorus, 116. 

Plating, gold, silver nickel, copper, 109. 
Polarity indicator, 107. 
Polarization, 80. 
electrical, 254. 
Poles of magnet, 2. 
Post-office box, 139. 
Potassium chlorate, 113. 
Potentiometer method of calibrating, 132. 



348 



INDEX 



Potentiometer, Leeds and Northrup, 133. 
Power in a. c. circuit, 273. 

factor, 275. 
Primary and storage batteries, 77. 
Primary coil, 11. 
Projection, experimental, 329. 

instruments, 340. 

lantern, 332. 

Queen bridge, 150. 

Rating of cells, 92. 

Reactance in parallel, 273. 

Relation of resistance and inductance, 263. 

Relay, 36. 

Remote control, 163. 

Residual magnetism, 52. 

Resistance, 59. 

car, 60. 

comparison with voltmeter, 128. 

measurement of, 125. 

of conductor, 62. 

of electrolyte, 143. 

of pure water, 91. 

of voltmeter, 22, 126. 

standard, 129. 
Resonance, 272. 
Retentivity, 7. 
Rolled negatives, 139. 
Rotary ammeter, calibration of, 131. 
Rotary converter, 50, 307. 

e. m. f.'s, 307. 

hunting of, 313. 

starting of, 309. 

starting from a. c. side, 311. 

starting from d. c. side, 309. 

starting with induction motor, 311. 

synchronizing of, 315. 
Rotating standard, 229. 

three-phase, 307. 
Rotating field, 319. 
Rotation, of motor, changed, 164. 
of series motor, changed, 180. 
Ruhmkorff coil, 56. 

Secondary coil, 11. 
Self-induction, 250. 
Series motor, 174. 

control, 182. 

starting features, 184. 
Shelf negative, 99. 



Shunt motor, 152. 

directions for setting up, 161. 

tests, 170. 
Single-phase wattmeter, 279. 
Slide wire bridge, 141, 
Sodium, 112. 
Sounder, 36. 

Spectrum, obtaining of, 187. 
Speed and tractive effort series motor, 

177- 
Speed variation, 165, 168. 
Sponge lead, 113. 
Standard cell, 66-68. 

cell circuit, 134. 

resistance, 231. 

resistance method for caUbrating watt- 
meters, 230. 
Starting boxes, 160. 

box magnet arm, 162. 
Station transformers, 293. 
Stator of induction motor, 299, 
Storage battery, 89. 

charging, 93. 

formulae, 90. 

operation, 91. 
Stray field, 26, 55. 
Synchronizing, 315. 

with frequency indicator, 317. 

with voltmeter, 317. 

with synchronoscope, 316. 
Synchronous motor, 326. 

Tabulated lamp bulb, 207. 
Tangents, 268. 
Tanks for projection, 104. 
Tantalum lamp, 210. 
Telegraph fine, 35. 
Telephone circuit, 55. 

receiver, 17. 
Temperature coefficient, 63. 

coefficient positive and negative, 102. 

effect on magnetizability, 8. 

of arc, 186. 
Testing meter, 235. 

sets, 149. 
Thomson double bridge, 144. 

inclined coil ammeter, 41. 

inchned coil voltmeter, 41. 

inclined coil wattmeter, 42. 

recording wattmeter, 17, 54. 

recording wattmeter armature, 221. 



INDEX 



349 



recording wattmeter armature resist- 
ance, 222. 

recording wattmeter field coils, 223. 

recording wattmeter magnets, 223. 

recording wattmeter type C, 224. 
Three-phase power, 277. 

e. m. f.'s, 253. 
Three-wire system, 119. 
Torque of series motor, 177. 
Traction magnets, 44. 
Transformer, 10, 283. 

connections, 295. 

coils, 288. 

cooling, 291. 

e. m. f.'s, 284. 

experimental, 288. 

formulae, 277. 

losses, 287. 

lugs, 291. 

subway, 290. 

theory, 283. 

types, 287. 

ventilation, 291. 
Transmission key, 35. 
Trigonometric expression, 268. 
Trouble, location of, 168. 
Tudor plates, 96. 
Tungsten filament, 212. 

lamp, 209, 211. 
Two- wire system, 119. 
Type H transformer, 296. 



Ultra rays, 191 . 

Unity power factor, 319. 

Vector, 266. 

relation of inductance and resistance. 
268. 
Vibration, effect on magnetizability, 9. 
Volt, 48, 59. 

Voltage, low, obtained, 75. 
Voltmeter, 17, 19. 

calibration of, 132. 

method of measuring resistance, 128. 

Thomson inclined coil, 41. 

Wattmeter disc, 18, 54. 

indicating, calibration of, 136. 

on inductive circuit, 281. 

polyphase, 280. 

Thomson inclined coil, 42. 

Weston indicating, 42. 
Weston a. c. ammeter, 251. 

indicating wattmeter, 42. 

relay, 21. • 

speed tachometer, 18. 

voltmeter suspension, 127. 

voltmeter magnet, 5 
Wheatstone bridge, 137. 
Wire roller bridge, 142. 
Wireless circuit, 56. 



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LUPTON, A., PARR, G. D. A., and PERKIN, H. Electricity Applied to 

Mining. Second Edition. AVith Tables, Diagrams, and Folding 
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MAILLOUX, C. 0. Electric Traction Machinery. Illustrated. 8vo., 
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MANSFIELD, A. N. Electromagnets: Their Design and Construction. 
Second Edition. Illustrated. 16mo., cloth, 155 pp. (No. 64 Van 
Nostrand's Science Series.) 50 cents 

MASSIE, W. W., and UNDERHILL, C. R. Wireless Telegraphy and 
Telephony Popularly Explained. With a chapter by Nikola Tesla. 
Illustrated. 12mo., cloth, 82 pp Net, $1 .00 

MAURICE, W. Electrical Blasting Apparatus and Explosives, with 
special reference to colUery practice. Illustrated. 8vo., cloth, 
167 pp Net, $3.50 

The Shot Firer's Guide. A practical manual on blasting and the 

prevention of blasting accidents. 78 illustrations. Svo., cloth. 
212 pp Net, $1.50 

MAVER, WM., Jr. American Telegraphy and Encyclopedia of the Tele- 
graph Systems, Apparatus, Operations. Fifth Edition, revised. 450 
Illustrations. 8vo., cloth, 656 pp Net, $5.00 

MONCKTON, C. C. F. Radio Telegraphy. 173 Illustrations. 8vo., 
cloth, 272 pp. (Van Nostrand's VA^estminster Series.) Net, $2.00 

MORGAN, ALFRED P. Wireless Telegraph Construction for Amateurs. 
153 illustrations. 12mo., cloth, 220 pp Net, $1.50 

MUNRO, J., and JAMIESON, A. A Pocket-Book of Electrical Rules and 
Tables for the Use of Electricians, Engineers, and Electrometallurgists. 
Eighteenth Revised Edition. 32mo., leather, 735 pp $2.50 



NIPHER, FRANCIS E. Theory of Magnetic Measurements. With an 
Appendix on the Method of Least Squares. Illustrated. 12mo., 
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NOLL, AUGUSTUS. How to Wire Buildings. A Manual of the Art of 
Interior Wiring,- Fourth Edition. Illustrated. 12mo., cloth, 
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OHM, G. S. The Galvanic Circuit Investigated Mathematically. Berlin, 

1827. Translated by William Francis. With Preface and Notes 
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OLSSON, ANDREW. Motor Control as used in Connection with Turret 
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No. 1.) Net, .50 

OUDIN, MAURICE A. Standard Polyphase Apparatus and Systems. 

Fifth Edition, revised. Illustrated with many Photo-reproductions, 
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PALAZ, A. Treatise on Industrial Photometry. Specially applied to 
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Svo., cloth, 324 pp $4.00 

PARR, G. D. A. Electrical Engineering Measuring Instrtunents for Com- 
mercial and Laboratory Purposes. With 370 Diagrams and Engrav- 
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PARSHALL, H. F., and HOBART, H. M. Armature Windings of Electric 
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Electric Railway Engineering. V/ith 437 Figures and Diagrams 
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Electric Machine Design. Being a revised and enlarged edition of 
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PERRINE, F. A. C. Conductors for Electrical Distribution: Their Manu- 
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POOLE, C. P. The Wiring Handbook with Complete Labor-saving Tables 
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POPE, F. L. Modem Practice of the Electric Telegraph. A Handbook 
for Electricians and Operators. Seventeenth Edition. Illustrated. 
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RAPHAEL, F. C. Localization of Faults in Electric Light Mains. Second 
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RAYMOND, E. B. Alternating-Current Engineering, Practically Treated. 

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RICHARDSON, S. S. Magnetism and Electricity and the Principles of Elec- 
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ROBERTS, J. Laboratory Work in Electrical Engineering — Preliminary 
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ROLLINS, W. Notes on X-Light. Printed on deckle edge Japan paper. 
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RUHMER, ERNST. Wireless Telephony in Theory and Practice. Trans- 
lated from the German by James Erskine-IMurray. Illustrated. 
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RUSSELL, A. The Theory of Electric Cables and Networks. 71 Illus- 
trations. Svo., cloth, 275 pp Net, $3 .00 

SALOMONS, DAVID. Electric-Light Installations. A Practical Hand- 
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Vol. I. : Management of Accvmiulators. Ninth Edition. 178 pp . $2 . 50 

Vol. II.: Apparatus. Seventh Edition. 318 pp $2 .25 

Vol. III. : Application. Seventh Edition. 234 pp $1 . 50 

SCHELLEN, H. Magnetc-Electric and Dynamo-Electric Machmes. Their 
Construction and Practical Application to Electric Lighting and the 
Transmission of Power. Translated from the Third German Edition 
by N. S. Keith and Percy Nejinann. With Additions and Notes 
relating to American Machines, by N. S. Keith. Vol. I. With 
353 Illustrations. Third Edition. Svo., cloth, 518 pp $5.00 



SEVER, G. F. Electrical Engineering Experiments and Tects on Direct- 
Current Machinery. Second Edition, enlarged. "With Diagrams and 
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SEVER, G. F., and TOWNSEND, F. Laboratory and Factory Tests in 
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SEWALL, C. H. Wireless Telegraphy. With Diagrams and Fig-ures. 
Second Edition, corrected. Illustrated . 8vo . , cloth, 229 pp . . Net, $2 . 00 

Lessons in Telegraphy. Illustrated. 12m-o., cloth, 104 pp. .Net, $1 .00 

SEWELL, T. Elements of Electrical Engineering. Third Edition, 
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The Construction of Dynamos (Alternating and Direct Current). A 
Text-book for students, engineering contractors, and electricians-in- 
charge. Illustrated. 8vo., cloth, 316 pp %Z .m 

SHAW, P. E. A First-Year Course of Practical Magnetism and Electricity. 
Specially adapted to the wants of technical students. Illustrated. 
8vo., cloth, 66 pp. interleaved for note taking Net, $1 .00 

SHELDON, S., and HAUSMANN, E. Dynamo-Electric Machinery: Its 
Construction, Design, and Operation. 
Vol. I.: Direct-Current Machines. Eighth Edition, completely re-ioritten. 
Illustrated. 8vo., cloth, 310 pp Net, $2.50 

SHELDON, S., MASON, H., and HAUSMANN, E. Alternating-Current 
Machines: Being the second volume of "Dynamo-Electric 
Machinery; its Construction, Design, and Operation." With many 
Diagrams and Figures. (Binding uniform with Volume I.) 
Seventh Edition, rewritten. 8vo., cloth, 353 pp Net, $2.50 

SLOANE, T. 'CONOR. Standard Electrical Dictionary. 300 Illustra- 
tions, 12mo., cloth, 682 pp S3. 00 

Elementr-ry Electrical Calculations. A Manual of Simple Engineer- 
ing ^Mathematics, covering the whole field of Direct Current 
Calculations, the basis of Alternating Current Mathematics, Net- 
works, and typical cases of Circuits, with Appendices on special 
subjects. Bvo., cloth. Illustrated. 304 pp Net, |2.00 

SNELL, ALBION T. Electric Motive Pov/er. The Transmission and Dis- 
tribution of Electric Power by Continuous and Alternating Currents. 
With a Section on the Applications of Electricity to Mi7iing Work. 
Second Edition. Illustrated. 8vo., cloth, 411 pp Net, $4.00 



WEBB, H. L. A Practical Guide to the Testing of Insulated Wires and 
Cables. Fifth Edition. Illustrated. 12mo., cloth, 118 pp SI -00 

WEEKS, R. W. The Design of Alternate-Current Transformer. 

New Edition in Press 

WEYMOUTH, F. MARTEN. Drum Armatures and Commutators. 
(Theorj' and Practice.) A complete treatise on the theory and con- 
struction of drum-winding, and of commutators for closed-coil arma- 
tures, together with a full resume of some of the principal points 
involved in their design, and an exposition of armature reactions 
and sparking. Illustrated. 8vo., cloth, 295 pp Net, $3.00 

WILKINSON, H. D. Submarine Cable Laying, Repairing and Testing. 
Second Edition, completely revised. 313 Illustrations. Svo., cloth, 
580 pp Net, $6.00 

YOUNG, J. ELTON. Electrical Testing for Telegraph Engineers. Illus- 
trated. 8vo., cloth, 264 pp Net, $4.00 

ZEIDLER, J., and LUSTGARTEN, J. Electric Arc Lamps: Their Princi- 
ples, Construction and Working. 160 Illustrations. 8vo., cloth, 
188 pp Net, $2.00 




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